Good day, and welcome to this group.
Over the years of working with relays of all kinds, including some very small signal relay like those used in RF test equipment, I've run into several occasions where a relay would develop a "perm."? "Perm" is a term that when associated with relays refers to some residual, permanent magnetism in either the pole piece or the armature.? It doesn't take much in the way of direct contact of the armature to the pole piece to cause a relay to not release when a perm develops.
In the past, and if it was possible to remove the relay, I would pass it slowly through an AC field, such as the type that was used to bulk erase magnetic recording tape.? This would usually cure the problem... for a while.
High quality relays will often have a piece of non-ferrous material in the armature where it contacts the pole piece.? This is to prevent the ferrous portion of the armature from directly contacting the pole piece.? When I first learned about this issue, I was able to put a piece of very thin piece of tape, such as the type of tape used in the olden days of splicing tape used in magnetic tape recording editing, across the pole to prevent the direct contact.? Obviously this can't be done in sealed relays.
The purpose of this epistle is to explain a common problem of why a relay might hang up.
Burt, K6OQK
Burt K6OQK