Re: sims 4 dine out staff quits after every single shift.

Have a look at this page:
http://sims4.crinrict.com/en/2016/06/restaurant-employees-and-culling/

There is a thread about it in the bug forums as well:
http://answers.ea.com/t5/The-Sims-4-Bug-Reports/WORKAROUND-Dine-Out-employees-not-carrying-over-from-shift-to/m-p/5383169#M25040