Have you got thermostatic valves (the ones that look like a plastic cup upside down)? If so, the valve could be sticking. Undo the knurled collar below the 'plastic cup' and lift off the plastic bit. You will see a little pin about 1mm in diameter sticking up. Tap it VERY GENTLY with a small hammer or similar blunt instrument. You should see it pop up a few mm's. Screw the plastic bit back on and (hopefully) it works.