This happens with my hot tap on the washbasin. The tap will not open fully. What i did was dismantle the tap to get at the valve. This is simple. Do not worry. Turn off the hot water supply. Plug the drain hole in the bath. When you have exposed the valve, take off the small open washer at the base of the valve. There will probably be lot of build up of gunge. Give it a good clean up. Give it a spray with with WD40 or clean it with a fine penetrating oil. Without reassembling it try opening the valve using the tap handle. I think you will find you can fully open it. Reassemble.
It worked for me.