How can i fix my Ariston A23 Mffi E Combi boiler NO DHW URGENT?

No Dhw, runs cold after a few minutes when demanded either alone or with CH demand, the burner continues to run and works fine, the CH works fine but warm only, the flow pipe from the primary heat exchanger is just luke warm used to be hotter before.

The diaphrams for the dhw and flow switch were recently replaced, both the flow and microswitch are ok and not sticky, diverter removed and cleaned out, spindle operates correctly to engage the mswitch, the bypass actuator also cleaned and reset. Pump works fine and has a fixed speed.

The secondary heat exchanger removed and cleaned and no leaks but seems to be cold from both ends when dhw demanded, seem sthe primary hot water flow does not enter the the secondary exchanger although the primary pipe are only luke warms so are the rads.

Could anyone please advice what other checks i could preform and why the primary flow water is not hot enough even with constant burning and why it does not enter the secondary heat exchanger to enable dhw through the taps??