@Junan I've also been having this problem since the last patch. My sims can't hire any staf for their parties: caterer, mixologist, entertainer, or (there's one more role, I think, but I forgot...). The household knows many highly-skilled sims who could be hired, but no one is available.
So, I give my home chef the "Always Welcome" trait, then make the household sims cook, mix drinks, and so on.
When I try to hire a mixologist, for example, the Staff Hiring menu looks like this:
Grrr.... *shakes fist