Do you have a directory with the phone numbers? If so, I would call and ask if they can make it. If not then email. It is so frustrating. I had a halloween party last year and invited the entire class...2 came. Funny thing is those moms have become my great friends and I've noticed we are the same ones that show up to all invites. Luckily I had some other good friends and neighbors that came so we had a great party. So, this year for my daughter B-Day I only invited very close friends and the two moms that came to the halloween party. Everyone but two came (one was having her son's first b-day and the other had a sick child....although she failed to let me know she wasn't coming and I found out through face book).
Good luck! Definitely contact them somehow so you can plan everything.