Hi,
You may have more going on than what I'm going to address, but let me start by saying, restaurants can be problematic.
I had downloaded Chez Lama, and when I place it, I discovered that I placed it backwards so that you had to walk around the back to enter the main entrance. I eventually had to re-build and call it "Casa de Lama."
In the rebuild, I opened up pathways, so wait-staff can get to and from the kitchen without bottlenecks. I learned that the placement of objects connected to the cooking and serving process had to be re-evaluated: the trash can was outside and around the building, so any time the waiter bussed the table, he'd get lost in the backyard.
Also... I discovered I had to eliminate the activity objects that aren't directly associated with food service/preparation. The NPC sims otherwise will become distracted during your meal and go play darts, for example.
Also, if you've just placed a new restaurant (without ownership control of the staffing), I think that the sims assigned to the lot have to develop / train in the skill set associated with food service before they become proficient.
Look. that's just my take on it. I could be completely wrong.
Have fun.