I have been having this problem too. But the weird thing is, when I hire a specific sim to be the caterer or mixologist or whatever, when I return home from the wedding location, the sim that I specifically hired is outside my house. As if they got called to my house instead of the wedding location.
Have begun to notice this with the December patch, sometimes the NPC will come incredibly late, and even then is still subject to the other issue of not doing what is required. If the NPC doesn't arrive late, then they won't appear for the duration of the social event.
(See: http://answers.ea.com/t5/The-Sims-4-Bug-Reports/OPEN-Several-Issues-with-Caterer/m-p/3446153#M13 )
Edit: Only seems to occur with 'hired' NPCs. Known NPCs arrive, along with their buggy autonomy and desire to cover my house up to the roof in unprepared meals. 😃
