Is the mixer tap new?
If new, the most recent taps now have a system within which reduces the flow of the hot tap. This is to stop people leaving on the hot tap and letting it run, thus reducing waste hot water going down, litterly, the drain.
If an old tap and only recently started doing it, the internal hot tap valve may need replacing.
Many of these mixer taps no longer have rubber washers and the unit has now a nylon type fitting. Unfortunaltely, the whole internal brass valve unit has to be replaced, Although not expensive, ensure you obtain the correct type of valve for that particular tap.
Davymac
July 2009