They may not know how much the slop is, or the exact procedure to open it if you do know the combination (turn left X times, then turn right Y times...). Though you'd expect a locksmith with access to a safe-cracking robot would also have access to documentation showing that.

Or as others said, the lock mechanism could be faulty or seized.

The lock might be broken? Gummed up lubricants come to mind.

No idea if that's a common problem with safes, but it's pretty clear that a safe would not have any outside maintenance access to fix a problem like that (other than drilling it open, which seems to be routine enough given the "yeah, we can make it usable again" offer)

The dial spins but the locking mechanism is rusted so you can't tell when you've hit the correct combo.

1. Faulty lock mechanism due to aging and wear.

2. It's a trap lock and was never designed to be operable in the first place.