Weird thought - make sure you don't have a case where all the townie sims are getting stuck somewhere. (like in
this thread - all the sims keep going back to one particular lot with no thoughts or cares for their own needs, even to the point of starving to death, even after the lot is bulldozed.)
Every once in a while I think just randomly you can end up with zero of your invited guests showing up, but for it to happen twice in a row I do think something odd might be up.
- Try the "resetsim *" cheat to reset all the sims in the neighborhood.
- Use the phone to invite just one sim over and see if they come, and if they do come if they stay.