Heating working in maintenance mode but not on demand

[Rado123]

Worcester junior boiler 5 years old. Hot water tank system. Suddenly stopped heating on demand, fires up for a few seconds then cuts out. Still heats perfectly in maintenance mode on Max and Min. Gas safe guy said it must be Tado thermostat (he doesn't trust digital things lol). I bypassed it - same problem. Came again and sad it's definitely danfoss actuator on heating line. Changed it - same problem. Pressure ok. Radiators all bled. Engineering says nothing wrong with the boiler. I suspect maybe valve is sticking? We have to heat the household manually on maintenance mode. Hot water only with immersion back up. Where is the problem guys? Thank for help