1. The latest version.
2. Random occurances of NPCs getting stuck outside the shop and never leaving, therefore they are forever a customer, so you never get new ones, and you have to restart the game in order to fix it.
3. (none available atm, I will try to get one next time it happens.)
4. a) Push a customer outside after they come in.
b) Wait a decent amount of time, it will happen eventually.