It's not a bug. Initially they aren't going to use the glory hole at the start because they don't know you're even there. At the start you gotta wait for someone to be in the stall next to you and start masturbating. She gets louder the closer she is to coming, and once you are loud enough they will notice you and more likely than not stick their dick in the glory hole.
Once you get some reputation in the toilets though you don't need to do this, as the prisoners now you are there and willing to do it, they will go to the bathrooms looking for Karryn working the gloryhole.
On a side note though, riots on floor 2 do lower the spawn rate of people coming to use the bathrooms, so if you have low reputation and there is a riot changes are no one will show up.