There's a connection issue with the tiny pins that connect your LCD to your motherboard. From experience, try to determine if you need a new motheboard or LCD if you can before you start buying replacement parts.
I'm having the same issue and this is my 2nd LCD/digitizer I bought off of Amazon. The issue is the connection from the LCD and motherboard; which is located right underneath the LG logo. In addition, after reading some other forums on this issue, there's some software issues when you install a non-LG LCD onto the phone.
There's so many non-Apple manufacturer's out there that it could be any set of issues. Low quality adhesive or misalignment of the bezzle has been my issue but most Non-Apple LCDs will work fine. And I'm surprised how fast the i6 and i6+ LCDs have came down in price without sacrificing quality. I think Apple's low $100 repair price point has something to do with it.