I get intermittent hot water with my bath mixer tap but perfect hot water from kitchen & bath sink mixer taps?

I get intermittent hot water with my bath mixer tap but perfect hot water from kitchen & bath sink mixer taps. It all runs from a combi-boiler and until recently all was ok, now there hardly a need to use the cold tap in the bath as the hot tap tends to only deliver luke warm water. All other mixer taps in the property are working fine?!?! I gather it's not a boiler issue but don't know what to do with the bath tap to fix it? Cheers