Boilermate not responding properly to CH call

Hi,

I have a valiant boiler downstairs and a boiler mate system upstairs. Our hot water is always fine but the CH is very hit and miss. We will have the thermostat set to 20 and the heating comes on and off fine. Then it’s as if the boilermate doesn’t receive the message that the temperature has dropped so nothing kicks in until we manually reset the temperature gauge on the boiler and then it will kick in and send hot water to the CH. it either goes straight off again or works for a bit and then the same issue arises. The thermostat is new and has been replaced. The boiler is working fine and was new last year so I’m assuming the issue is the boilermate but we have asked 3 plumbers to look, 2 never came back or answered messages, and the other is a family member who has no idea!
Any help would be much appreciated. Why does the CH work when we reset the boiler but then the thermostat kicking in due to a temperature drop works intermittently?