need to see what the problem is first.

if the valve stem is getting stuck, it could be the profile of the mushroom head, somehow settling into and jamming into the valve face itself. But that would be unnusual, unless it's been previously modified, or made from a dodgy batch.. the material is delrin like. Maybe it could save some grease of something sticking in there ? Or an excessively heavy valve return spring. It's quite weird.. see if a few taps frees it up first.

Then again, maybe it could be as simple as a bent stem.. trouble is, if the stem was that bent, I wouldn't expect the head to seal. and those stems are thick / heavy duty as standard.

Puzzling.