You then need to determine what it is causing it to move very little...
I would say it's either a stuck valve or worn lobe on the camshaft... Why is it moving very little is the question that you have to find answer to....
Remove the rocker arm and tap on the valve with a hammer and see if the valve will open more with a hammer or if it gets jammed in the guide... If the valve gets stuck in the guide, then it most likely will be caused by a bent valve, oversize valve stem, or undersized valve guide hole...
If the valve moves freely and opens more than 'very little' then check the camshaft for a worn lobe...