If the flow of water is ok when you open a hot tap, then I'd check the pump and flowswitch first - does the pump come on when a hot tap is opened ?
If the flow of water is not too good from the hot taps - then could be blocked plate heat exchanger - are you in hard water area?
If the flow of water is not too good from the hot taps - then could be blocked plate heat exchanger - are you in hard water area?