Touch Screen (digitizer) tempramental after new one fitted?

Firstly I have now successfully removed and fitted over 50 new digitizers on Ipad's including 10 iPad mini's.

The one in question was bought to me after a failed DIY fix. I have attempted to fit 3 different digitizers that work perfectly in other machines but without success.

The problem is very temperamental, sometimes the touch screen will work perfectly and at others not. I have attempted to apply pressure at different parts of the PCB without change, the ribbon clips and connectors look ok under magnification and I have restored the devise back to factory setting and updated. Nothing makes any difference.

Be grateful for any help and advice - many thanks Dan