only change up to 80% I read a few places (including the instructions that came with the juice pack plus) that after 80% it takes longer and needs more power to change your phone. So after 80% shut off the juice pack, and don’t wat for the phone to die, start charging it at 20-30%, and you’ll get more out of the external battery.
Also you're probably using it as it's charging, so that will also lessen the battery.