OK, so check the charging port to see if there is lint or other debris that is prevent the charger from making full contact:
- Using a clean, soft, toothbrush, brush out the port on both the phone and the charger
- Many posters have reported that using a wooden toothpick on the phone port and carefully digging around the corners has produced copious amounts of lint. If you keep your phone in your pocket, this is very possible. Try cleaning out the phone port thoroughly
- Also, you need to try a different USB cable to make sure it is not the cable itself that is causing the issue. Be sure to only use an Apple cable or an Apple-certified cable. Other cables will not work.
Best of luck,
GB