The cost of those return visits has well-exceeded the cost of replacing the lock outright. Unless there is some level of local interference (not common, but I have seen this a time-or-four), that lock should have been replaced and should be operating flawlessly.

Now, on a couple of occasions, a local machine in operation stepped all over the locks nearby. One being an open MRI machine, the other being a defective hyfrecator. Both cases were solved with by installing shielded locks (and the hyfrecator was also replaced, of course). Do you have any unusual equipment within about 20' of that lock, especially something that is intermittent in use?

Peter Wieck
Melrose Park, PA