Hi,
For initial use, every new battery needs to be charged for 12 hours.
Please also perform 6 full charge/discharge cycles to ensure battery optimal performance.
If the above steps are not done as shown in the instructions, it may cause negative performance results.
------------------------------------------------
When the issue happens, please check if any background app is draining the phone's power. You may turn off the Bluetooth or other unnecessary programs, to increase the charge speed.
Also, it is greatly depending on the charger output and phone input.
If possible, perform a factory reset on your phone to see if the situation improves.
Here is an XDA forum thread explaining why the battery percentage on the phone falls rather quickly sometimes and how to resolve this problem.
http://forum.xda-developers.com/showthread.php?t=871051
If the problem still exists, we are happy to offer you a warranty replacement, please send an e-mail to info@zerolemon.com with your order number. Thanks.