How can I get my pottery customers to finish their projects?

So, in general, I like the new Hobbies and Business add-on, and my second-generation sims moved out of Mom and Dad's house and opened a pottery business. But how do I get the customers to finish their projects instead of leaving them on the pottery wheels? So far, only the owners of the shop can't fire the clay, and customers leave their projects on the wheel, and I can't sell what they make! 

(By the way, this is the first time I managed to get to a second generation in the game!  The third generation just got born!)