I've had this happen before in the "attic" space of my house. Took me forever to find it and finally figure out the reason. In my game it was due to dust build up (from Bust the Dust kit). The attic area was just for show (to give me dormers), I never thought dust would be up there, since there was no access and no one ever went up there. After many times of this occurring, I finally just put a robot vacuum (fully upgraded) up there and never had an issue again. You could also turn the dust kit off in the settings. I hope this helps.