Extremely low hot water flow rate on Sabre Combi Boiler?

I have a Sabre Combi Boiler in a 3 bedroom flat on the 2nd floor of a low rise block. Cold water flow rate is very high, but the hot water flow rate is just a trickle. The Sabre Combi boiler provides good heating for the radiators but negligible flow rate/pressure for hot water from all taps. What could the problem be here?
I don't know the model of the boiler.