Why are some rooms not being cleaned by staff?

It’s not a bug. They want it like that so you watch ads. It’s how they can make profit.

This does happen to me, but just restart the app and it should work fine again. It worked for me.

There are upgrades you can do within housekeeping that allow the staff to clean rooms faster. A combination of hiring more staff and upgrading the cleaning equipment should allow you to keep up with the dirty rooms. But if all else fails, you can click on a dirty room and watch an ad to clean all dirty rooms at once.