How can i fix my shower pressure?

When ever I turn the shower on the hot water drips out but the cold water comes out fine?? I've had a plumber over, the boiler is brand new, the facet was cleaned and there doesn't seem to be any other problems. We've evened turned off the central heating, not used any other hot water in the house and nothing has worked...
Does anyone have any idea how I can have a hot shower with the hot water coming out properly?
(p.s. Its not an electrical shower...its gas)