Sounds like a worn cam or tappet. Take the rocckers off and pull out the followeres. If they are hard to get out then more than likley the cam lobe is worn on that valve. They should be flat and if they are dished they need replacing along with a new cam. Dont put a old tappet on a old cam because they wont be matched and will wear again. Also with the tappets out press the button on top hard to see if there is and movement. They all should be quite hard. If you get a springy one then the tappet is not filling up with oil correctly and is no good.

The followers were desined to turn in the head and if they dont turn then the wear in one spot. Get a friend to help you and Disconect the coil lead and crank the car over. (with the rocker cover off and rockers done up ) While your mate is turning the car over look at the tappets and see if they turn. If they don't turn then that is a dodgy tappet.