are you sure the customer is selecting programmes where it allows you to use them,ie: it will only allow you to select an option which is allowed for that programme, which will explain why it is only working sometimes.
I had this exact fault with a CM2 machine. She couldn’t select short wash and the other buttons didn’t work either. I found the ribbon type cable from the panel had been rubbing off something and eventually worn through the cable. I’d take out the cable and check it. You need to look thoroughly as the wires are small.
The start /pause board is one of only two items which come up when you type that model into connect’s search box. It’s o.o.s. but only £7.61+.
Could be worth a try.
Mike.