hot tap in bath water doesnt get hot , its warm unless tap is juts slightly open

You definitely have a restricted hot water flow through your tap. This can be confirmed by opening only hot water, it's flow, compared with that of a cold water, would feel much slower. To diagnose it I'd remove the hot water pipe from the tap and I'd check the hot water flow from it without tap, if it's strong enough, the culprit is in the tap. If it's not, pipework from the last properly working hot water tap needs to be carefully inspected for dents, old isolating valves etc. Also, even if there nothing suspicious, it happens that some foreign debris may be blocking crossection of the pipe causing water flow to drop significantly. Hope it helps! Vidas

Maybe thermostatic valves not working properly

Could be a faulty thermostatic mixer tap, or potentially a booster pump on the hot water supply to the bath that has failed or not even powered on.