OK...purchased iPad Air 64gb LTE model a week ago. 90% battery out of the package. Used for about a week and was down to 25% (didn't use so much, and battery seemed pretty robust to me based on amount of use, time that went buy and amount of discharge). Plugged in and same issue as everyone else: Didn't charge very fast (3 hours to get from 25%--->40%).
Called Apple Care, got call escalated, got a senior tech support guy, and got the problem solved quickly. Here's the quick fix: Gotta use the 10 watt plug that comes with the iPad Air, and not the smaller 5 watt plug that comes with the iPad Mini, iPhones, etc.
Turns out the iPad Air battery is pretty heavy duty and needs the heavier duty charger. Immediately started charging much faster (40%--->69% in 45 min which seems pretty reasonable). Apple Care guy really nice...understandable not to thought the new charger was that important, but it is. The iPad mini can use the smaller one the phones use...iPad Air not so much. See if that fixes your problem before throwing in the towel, throwing the iPad under the bus, returning the thing, resetting, Apple Store appt, etc, etc. Hope this helps.