I was going to say too maybe the parents are a bit shy, I know I stand back a bit because the other parents that do chat seem to know each other well.
As for the party its so hard. I would let it go, it is sad tho, I know I feel upset if DD misses a party. But then at the same time we are organising her b'day party and we can't invite everyone and so many people she considers close friends.
Ask a variety of kids over for playdates maybe, mix with different kids.