Let me tell you why you haven't heard back from parents. It's because they are rude and inconsiderate and can't take the 60 seconds to respond. There, I've said it.
Before anyone blames my attitude on what happened with DD's recent party, let me explain that my attitude stems from observations I have made over the years regarding more parties than I can count. The pattern is the same. The mom sends out invitations. Maybe they get send home in a backpack, in the mail, by email......It matters not. Maybe she includes her phone number, her cell phone, her email or
every method of contact in the world. It does not matter. Maybe she gives an RSVP deadline of a week before the party, 5 days before, 3 days before, or even the day before. Again, it does not matter.
People will ignore the RSVP no matter what.
Yes, some are truly ignorant of what RSVP means. But I can promise you where I live, they are not. Nearly everyone in this town has a college degree and learned what RSVP meant somewhere along the way. And as I told DH, if their husband's boss sent an invitation home, you can bet you rear end they'd respond fast enough.

No, it's something else. Maybe they are waiting to see if something better comes up or maybe they are just plain old inconsiderate. I don't know. But I do know you cannot count on people responding to an RSVP even if you say, "Please let me know if you are coming or not because I need to give the venue a head count by X date."
What happened at DD's recent party is typical and has happened to many people I know. We didn't have a directory yet or a class email list. The school is very strict about privacy rights, so sending invitations home in a backpack is the only way to get them to the kids. DD's teacher made certain every girl in the class (10 girls) got an invitation in their backpack. Two girls in other classes received them as well. I gave them 2 weeks before the party and needed a reply the Tuesday before a Sunday party.
On Monday, when I hadn't heard from anyone, I told DD that if I didn't get any responses, we needed to cancel the party, because what was the point? She was upset and said the girls were saying they were going to come. I told her there was a reason the RSVP said to reply to
me, the mother and that I needed to be contacted. In the past, I have learned that when a kid says they're coming, it's not all that reliable. I told DH if they showed up for a cancelled party, gift in hand, it would serve them right.
On Tuesday
NIGHT I received the
FIRST reply. Yep, on the deadline, I received reply #1. The venue had already called wanting a head count and I told them the situation. What could I do? And you could not misinterpret the RSVP itself. It said something like, "RSVP by Tuesday, Oct. X to EMom at phone number or email." Starting on Wednesday, the day AFTER the RSVP deadline, the RSVPs began rolling in. Actually, I'm surprised they even responded. (I had included my phone and email.)
By the time it was all said and done, I went from one pitiful response on the RSVP deadline day to having 8 out of 10 girls in her class attend. Of the 2 girls who were not in her class, both attended......But one showed up having never RSVPed, so it's a good thing I brought an extra goody bag. The RSVPs continued to come in until the night before the party. I could not believe it.
There I was, wondering if DD had suddenly become unliked and without friends since no one was calling, and of the 12 kids invited from school, 10 showed up.

I really think I've had it. It was just too stressful, not knowing if anyone was going to show up until virtually the last minute. Crud, the year we took her out of school and took her to WDW for her birthday was less stressful than that.......More expensive, but less stressful.
I told her next year, we may take her somewhere (maybe let her pick a friend to take) like Renaissance Festival and let her spend to her heart's content. Whatever we'd have spent on a party, let her have a go-wild day with that much money. I seriously think I am done with birthday parties.