Whenever I enter the dinner party at 6:00 P.M., she doesn't appear. I've tried reloading multiple saves and doing everything I can think of, but she just won't appear anywhere. Can I have some help?

Nevermind, she just randomly reappeared.

Sometimes when that happens, you might have to wait through another day for her next dinner party.

She also goes off on a two or three day trip to visit her mother once a month. During that time she doesn't show up for the dinner party. She can get killed on that trip IIRC, after the Thieves' Guild Quest where she's essential.

Closed by request of the OP.