Are you SURE it is not opening? If so take the spark out and close the gap completely.. Then reinstall and see if it runs better on 5 cyls. If your lobe is that worn you do need a new cam and lifter set. This will involve more work than you might wish to do however.
You must have someone turn the engine by hand to see how much the valve is opening.
I have never seen a situation like you describe that the lifter did not complain loudly.
At rest and the valve closed sometimes there can be slight amount of gap between the rocker and the valve/pushrod as the lifter has relaxed. However under pressure the gap quickly closes.. A feeler guage can be inserted between the valve stem and the rocker while ideling to see if opening the valve farther will help the idle.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />