The thing is when our dc don't get many party invites it isn't necessarily anything they've done or the parents/other children being mean. I've had 3 dc and they all operated differently.
Dd1: Got every party invite going-even those she wouldn't have called friends. She was often the only, or one of the only girls at a boy party, and got invited to both her class and the other class' girls parties. She sometimes had 2-3 at a weekend.
Dd2: Hardly got invited to any except her best friend.
Ds: Gets invited consistantly to a small group, but very rarely any others.
Actually dd1 would have said she had the smallest circle of friends and is the most shy and the least inclined to socialise from a not terribly socialble family.
But she was quiet and kind, and I suspect she ended up being invitee number 8-10 on most people's list, but was number 1-2 on few.
Dd2: I could blame this on her special needs. She's missing a hand though, and I doubt it. There were parties where she was one of 2 girls not invited and things like that. What turned out (some people came to apologise) was that she had a very similar name to another in her class. For some reason the class called them both by the other girl's name-the girls involved thought it quite funny, and so parents didn't realise it was actually two girls-and if they did the invitation would be labelled with the other girl's name so it went to her. But in this happening, she got sort of forgotten because parents saw less of her at parties.
Ds: Is lively and excitable-he has a small group of close likeminded friends, and he reliably gets invited to theirs. He almost never gets invited to others-but he doesn't really mind. The only time he got upset was when he hadn't been invited to one girl's party whose invites went out before the holiday (Easter) and their friendship had only bloomed after the invites. Wouldn't have dreamt of saying anything to mum though.
Equally well playdates: I don't do many, partually because evenings tend to be a little hectic with 3 dc, two of whom have homework, and all 3 have clubs some evenings. Plus ds in particular is too tired and emotional when he gets home really. He needs down time and time on his own. He doesn't really enjoy the reality of going to play, although he loves it in theory. So I have refused playdates for him on that basis.
Dd1 also used to not always want to go. Not because she didn't like the other child, but she would get so wound up and scared, she didn't want to. If I knew the parent well then I'd get her through it, but if I didn't, yes, I did refuse (politely).
Dd2 would go to any happily.