One reason might be the connection to the boat's bonding system. As was explained to me by the former director of engineering at a major boat manufacturer, the current flowing to the zincs is miniscule. So it takes very little resistance to slow or stop it.
We had a similar problem, one transom zinc going away normally and one not at all. I checked the continuity of the boat's bonding system to and between the zinc mounting studs (inside the laxarette) and found that all was fine with them. That's when I mentioned this to my friend.
His suggestion was to first clean the mounting studs for the zincs with bronze wool to remove any corrosion, coating, scum, whatever, that could create resistance to this tiny current.
I did that, and that was the end of the problem. Today, when I change the transom zincs (I can do this from the dinghy, for the shaft zincs we use a dive service) I clean the bronze mounting studs with bronze wool along with the nuts and washers that hold the zincs in place. And we have never had the problem of a zinc not going away normally again.
So you might try this before moving on to checking the boat's bonding wiring, connections, etc.