Several possibilties that come to mind. 1) Stuck valve, real easy to rule out. 2) Valve spring that is way to stiff. 3) Weakened pushrod due to fatigue. 4) Valve lash set way to tight. 5) Detonation with the valve partially open combined with a weak pushrod. Most likely. Don't know what your plans are for the engine, but unless it's future includes hard use, I would get a pushrod out of another 216 to replace it.
