Hi,
This might be an effect from poor contact, please try to remove the phone from the battery case and wipe clean the battery metal connector, then put the phone back. Try to push a little hard from the battery bottom to make sure the phone sits tightly in the battery case.
If the problem still exists, we'd love to provide you a warranty replacement. Please send an e-mail to infor@zerolemon.com with your order number. Thanks.