Touch screen not recieving touches

I have an official 7” touchscreen with RPi 2 and HiFiBerry Digi+. All working well for the last 2 years or so. Recently the screen has stopped responding to touches in increasingly large areas. I have dismantled and checked all ribbon connectors, and updated to all the latest software versions - to no avail. I have also used SSH to run ‘evtest’ to test touch events generated by the screen - this is generating events but only in the remaining touch sensitive areas which are now confined to top right and bottom right hand corners. I am thinking this is a hardware issue or something to do with the capacitative touch functionality but have run out of ideas as to how to diagnose or treat. Any help gratefully recieved.