This always happen to, when my Flash2 hit the 75% it stop to charge, and also it super slow to charge when i charge it from 20%, and also there's have a time when i hit 50% or sometimes 30% it begin to drain very fast it's like a time every second the % will drop
Are you on Marshmallow or Lollipop?
please try calibrating your battery,
going totally 0% then recharging the phone (while turned off) back to 100%.
For slow charging, try a another charger