I have a bath mixer tap and when you turn on the hot water tap the water just trickles out and then stops completely. The cold water is fine and has full pressure, likewise both the hot and cold mixer taps on the sink work fine. The heating and hot water operate from a combination boiler, any suggestions?
If you unscrew the 'handle' part of the tap (hot side) off, there should be a barrel you can unscrew inside (after turning water off) which when turned via the handle, opens a 'gate' within it which the water flows through. This can be blocked, restricting or preventing flow altogether. I changed mine in the kitchen mixer tap recently. The guy at the plumb centre referred to them as 'quarter turns' - presumably because they turn through 90 degrees to open...
If its a thermostatic mixer tap it may have a hot and cold inlet and these could have been connected wrongly when it was fitted, i had the same problem apart from it being a thermostatic mixer shower i swapped the hot and cold inlet around and it worked well