Pressure differential valve not opening - vaillant combi

[savolover]

hi, have been a lurker on here for a while but only signed up today to post. I have a vaillant vcw 20/1 th3w which has recently stopped firing for heating and hot water. The pump is running, i have a pilot light but the pressure differential valve pin is not pushing the gas valve. there seems enough pressure to push it but i'm not sure how much is required. I've read a few other posts pointing to the servo valve? or the water section diaphragm? Any ideas? Thanks in advance