There's currently two issues (although they could be connected).
Please use this thread if your bartender doesn't stay behind the bar but wanders off to socialize, leaving the bar empty. Use this...
Ideas
Just today I tried to have a date at the bar in Britechester and the Mixologist wasn’t making any drinks. Then she left the bar to talk to a guy and when I tried to command my Sim to make the drinks himself, she came back to the bar, blocking it.
The option to order came up, I ordered a drink, she left the bar to talk again to the same guy.
After a while I resetSim the mixologist and she got away from the bar for long enough for my Sim to make drinks for his date.
