I've probably made this reference once too often, but there's a joke that despite what the acronym stands for, USB isn't very universal. There have been many different connector types over the years, not to mention many different speeds, which can leave people scrambling for different cables, or wondering why a file transfer seems to be taking forever. It's better than what came before, or than Apple's defunct Lightning format, but that's not saying much.
A cable or port is physically damaged
Exercise extreme caution
This one is up top for safety reasons. If a cable's inner wiring is exposed -- that is, you can see the metal threads under the sheath -- it may or may not still work. But if you've established that power isn't flowing, it probably won't work again, and keeping it in circulation isn't worth the risk. There could be hidden damage that you can't see, and a short may lead to fire or electrocution. It's always safer to recycle an unreliable cable as e-waste. Jump straight to that step if the inner wires are frayed.
It's always safer to recycle an unreliable cable as e-waste.
While you're at it, check for anything that might be bent, corroded, or scratched, both on cables and the ports you're trying to plug things into. That could be the source of your problems. It's sometimes possible to bend things back into the right shape, but any other form of damage means you should probably stop using a port or cable entirely.
A port, cable, or charger isn't rated for the right amount of power
Confusing standards strike again
Particularly in the age of USB-C, it's easy to assume that any charging gear should be able to power just about any device that's halfway portable. In reality, however, some accessories may not have sufficient wattage. If you're trying to charge a 240W laptop, for instance, you'll probably need to use the adapter and cable that came with it, since very few USB accessories can handle that much juice. A 100W adapter will only slow down that computer's drain. The 30W charger that shipped with your tablet will do virtually nothing, in this scenario.
As a rule, USB-A connections are likely to supply less power than USB-C -- no more than 18W, and potentially as little as 2.5W.
Likewise, if you're charging to or from a port on a computer, car, or battery pack, you may need to doublecheck that you're plugged into the right one. Different ports can have different power ratings, even if they're using the same connector type. Some laptops may expect you to use a specific port for power input.
As a rule, USB-A (rectangular) connections are likely to supply less power than USB-C -- no more than 18W, and potentially as little as 2.5W. An iPhone plugged into a 2.5W port will take a very, very long time to charge.
A port or connector is too dirty
Rare, but not impossible
If a port or cable is left in the same place for a while, it's bound to accumulate dust and other debris. I've never had this interfere with charging before, but it's basic physics that if enough material gets in the way, electricity won't flow properly. You might also want to investigate any accessories that you pulled out of a pocket, purse, or backpack, since they could be covered in lint or grime. Be sure to clean any dirty accessories thoroughly -- you don't want to risk electrical hazards.
It's best to use an air bulb, a compressed air can, or a microfiber cloth, making sure that whatever you're cleaning is shut off and/or disconnected.
Speaking of which, some methods of cleaning are safer than others. If debris can't simply be wiped off with your fingers, it's best to use an air bulb, a compressed air can, or a microfiber cloth, making sure that whatever you're cleaning is shut off or disconnected. Avoid using anything sharp that could scratch a contact, such as a pin or toothpick. You'll also need to steer clear of liquids. In fact, when you're using compressed air, keep the can level to avoid spraying its fluid on electronic surfaces.
A component is simply defective
Time to put on your lab coat
While I've already talked about conspicuous signs of damage, it's worth calling attention to the possibilities of accessories failing without external evidence. You might be the victim of poor quality control, or the components in older accessories becoming worn out. Remember, cables and chargers aren't just stuffed with wires -- often they include chips for authentication and power regulation. Indeed if a high-wattage cable can't verify that it's properly rated, the item that it's plugged into may throttle charging, or prevent it entirely.
Try swapping in fresh cables or chargers, or plugging suspect accessories into different ports or wall outlets.
If you think a defect is a legitimate possibility, there's not much you can do but experiment. Try swapping in fresh cables or chargers, or plugging suspect accessories into different ports or wall outlets. You might even try plugging them into entirely different devices, just to rule out any issues with the gadget you're trying to charge.
Should your science prove that an accessory is defective, stop using it immediately and dispose of it as e-waste. It could be just as much a danger as something with exposed wires.
A device needs new software settings or updates
Computers can be more finicky than you think
On a computer, some settings may potentially prevent USB ports from receiving power when it's needed, like your UEFI/BIOS configuration, or USB selective suspend on PCs. In Windows 11, you can check the state of selective suspend by opening Control Panel and going to Hardware and Sound -> Power Options -> Change plan settings -> Change advanced plan settings. Under the USB settings tree, you'll find options to enable or disable the feature. Bear in mind that this may or may not be responsible for power issues -- I'd recommend ruling out other concerns first.
It could be that your computer needs a driver update to fix compatibility issues, or is somehow missing a driver entirely.
Remember also that computers use software drivers to detect and manage USB ports. It could be that your computer needs a driver update to fix compatibility issues, or is somehow missing a driver entirely. If your operating system's automatic update mechanisms aren't helping, you may need to identify your computer and/or motherboard model and install a driver from a manufacturer's website.
More rarely, it could be that a bug with a device's OS is causing trouble. This is one reason why it's important to update phones, tablets, PC, or any other device on a regular basis.