A stuck relay that releases on being tapped usually indicates that (some of) the contacts themselves are sticking together, presumably from arcing.
The only real cure with these small sealed units is to replace with a maybe beefier alternative, and in the longer term try and check whether the relay is in fact being commanded to open before the transmission has ended completely. Or maybe the transmission is starting some milliseconds before the relay contacts close properly.
For serious VHF/UHF work this what sequencers for the antenne and power switching arrangements are for, to make sure that the antenna changeovers are made before the signal starts, or after it ends. Thus ensuring the relays, and PA units, aren't subjected to unnecessary stresses.
For testing, one can make continuity checks across the contacts while is in the 'stuck' state to be certain of the diagnosis. But very often any slight movement or vibration will cause the relay contacts to release.
Of cause there could be a dry joint somewhere so reflowing all joints to the relay and its driver is an important first step. And see if the sticking still occurs at lower levels of, say, 10W.