I have this issue with some of the food and drink shops too. For reasons known only to them, vendors will refuse to work in some of them unless I delete and replace. This was an issue in one of the more recent career challenges. The game starts out with several shops, but if I hire vendors for them and plop them down in front, they refuse to work there and just walk away.
Sounds like the modular gift shops have been problematic, though, so I haven't tried them yet.