Hiu hot water issue- only warm water not hot

[Sonny Rowling]

I am working in a flat that only has luke warm hot water. I replaced the PM valve which seemed to fix the problem a month later ot has returned. I have cleaned the heat exchanger and ther dosnt seem to be a blockage. I have checked the thermostatic control valve all seems fine. When I flush the primary floe through the heat exchanger the hot water returns to full heat for a while. But the problem eventually returns. Dose any one have a clue what's going on ?
It's a duty point hiu there is no strainer.

 

[ShaunCorbs]

Sounds like the plate is blocked what’s the difference in temp on both sides ?