We have noticed quite a few iPhones coming in with charging issues recently and wanted to get some information out about the most common causes. Other guides may provide part of the solution, but we have yet to find anyone who explains all of the possible causes (except Jessa who is an exception to most rules).
Lint In Charging Ports can cause the phone not to charge. Ohh, the joys of pulling pocket lint out of iPhone charging ports. Although it is relatively easy to do with a pair of thin tip tweezers we strongly recommend relying on a third-party. There are two main reasons for this: (1) There are thin pins that can be damaged if not carefully avoided, (2) Additional diagnostics can be run on the device to verify that there is not another underlying issue contributing to a failure to charge issue.
Failing Batteries can cause a whole range of symptoms from unexpected shutdowns to a failure to charge properly. You can check the current health of your battery using a utility like Coconut Battery (http://www.coconut-flavour.com/coconutbattery/#faq) but there are more thorough options available to determine the current health of the battery which are not readily available to consumers.
Consumed Batteries frequently fail to charge as quickly as entirely functional batteries and in some cases can stop charging completely or simply not be able to provide the amount of power required to turn a phone back on once it is completely discharged. Most consumers hold onto their devices for long enough that their battery will enter a consumed state. Battery replacements are quite affordable
Damaged Logic Boards are a possible cause as well either because of a device specific component failure like the U2 Chip in many Apple Mobile Devices which is supposed to control power flow or liquid damage. Luckily damaged logic boards can be repaired, but you should expect a turnaround time of about a week due to the complexity of troubleshooting required to identify problematic components.