iPad Air says “Not Charging” sometimes but often

I apologize for the dumb question but this may benefit others who stumble upon this question. Which charging brick are you using? If you are using the small square one that is typically packaged with iPhones, then this is exactly what you will see. For the iPad, you need to use the larger 12W charger.

Now assuming you are using the proper charger, have you checked PP3V0_S2R_TRISTAR & PP1V8_S2R_TRISTAR? You'll also want to check the VCC_MAIN circuit (Q8104) as well as the voltage protection circuit (Q8123).