LG G3 Cat.6 — Touch screen unresponsive on LG G3 after screen wakes from sleep/lock (but fine before)

Firstly let me assure you I have searched google generally and also looked through lists on here specifically for an answer and can’t find one (some similar but not the same and don’t help). I’m not one of those lazy people who just posts a question without bothering to look for answers already on the site.

Please can you help me, I recently managed to drop my LG-G3 and smash the screen – it was cracked/shattered in several places, to the point I could still see the wall paper image below but the touch was unresponsive. Having viewed various video demos, I felt comfortable replacing the entire screen but wouldn’t want to mess around trying to separate glass etc. so spent £80 on an entire new front piece with glass, LCD and I presume, digitiser?

Using the correct tools (which I have successfully used in the past on another phone) I carefully removed the screen and glass debris then replaced the new one and screwed it all back together again. Initially it was fine however after the screen locks/sleeps and I wake it up by pressing the power button on the back it is totally unresponsive, meaning I either need to remove and replace the battery or as a I later found, keep tapping the screen repeatedly whilst pressing the back button on and off a few times to switch between screen on and sleep, after which it eventually unlocks (I had removed the knock code earlier).

Once I have control again its fine, with normal sensitivity in all areas, but as soon as the screen goes off I have the same problem.

I have tried updating all software updates and I have also carefully removed the two rear back plates and board to check the connections were firm and cables ok – they all seem fine but the problem remains.

Could this be a software issue or something else I could try, or have I been sold a duff screen and should ask for a refund/replacement?

I have added some information below you may require to advise?

Network = O2
IMEI SV (whatever that is?) = 07
Model number = LG-D855
Android version = 5.0
Baseband version = MPSS.DI.2.0.1.c1.13-00048-M8974AAAAANPZM-1
Kernel version = 3.4.0
Build number = LRX21R.A147604035
Software version = V20e-EUR-XX

Thank you for anyone who takes the time to read and respond – I have done quite a bit of assisting people on another site (in another area, of which I have some expertise) so I’m hoping for some Karma!