Neff T46FD53X0/01 Glass top replacement

Good Morning Folks,
I wonder if any body can shed some light on this issue, we were called out by our customer to replace a cracked glass top to a Neff T46FD53X0/01 Induction Hob, we duly arrived with a replacement glass top and fitted it. I checked it switched on and selected a hotplate and all was well. I was called back two weeks later with the customer complaining that she could only select one hot plate intermittently and could not select the others. I checked and the customer was perfectly correct, so I popped of the glass top and you can select everything if you touch the pcb directly all is well but if you put the glass top on you can turn the hob on and off without any issue but cannot select the hotplates. I have spoken to Appliance Care and they suggest it might need calibrating but I am having difficulty’s getting through to technical services (on there premium rate phone number). Has anybody come across this problem and can suggest a solution. Many thanks in taking the time to read this problem.

Regards

Neil