How old is dd? Did invites go out via school? if so, can you try and "bump into" other parents and ask them face to face if they are coming? It is really quite ignorant to not respond at all -are you sure all the invites went out?
You will probably find that there is a last minute rush, Don't cancel the party just send a brief reminder out. When my dd had her party some people didn't reply until the Thursday before the party, which really pissed me off. I don't think people mean to be rude, it just slips their minds.
How old is she? My patent trick (worked last year - Y2) was to send DS in with a list of non-respondees and tell him to tell the children concerned that if their parents did not reply there would be no party bag for them.