on the compacts there is a flow restrictor on the inlet to the heat exchanger,i have a few cases where it gets restricted wirh debris slowing down the outlet flow to your hot taps
Why is it on some vokera combis changing the dhw diaphragm is straight forward simply because its split,yet on others its because it seems to have overheated and melted inside the housing causing it to stretch and mis shape
I would say that if its aparking ok then aps has proved ok,check inlet pressure?Is there voltage to the gas valve?If so suspect electronic failure from pcb
with the case off and the boiler fires up could be a few things!The aps could be at fault or the aps tubes blocked or cracked.Is the heatexchanger clean and all the flue ways clear?