I find some of the invites are reciprocal. Certainly with DD1 when she had a birthday party, I found her party invites would increase. I do often feel obliged myself to invite in return, even though I am debating that this year.

DD2 took time to get invites, but once she started at childcare 5 days per week last year, the invites really jumped. Could also be due to 2 other girls having the same name and maybe the mothers just invited all three rather than miss inviting the right one