It is annoying - we put those (possibly tacky) but helpful "response" cards in our wedding invites to make sure that peoples rsvp'd us. Bit late for that I guess. Could you email the stragglers? Say something like,
"Hoping that you are able to come, please let us know so that we can confirm with our caterers" or something to that effect.
Your party
"Really Sounds Very Pleasant"
though
Hope you get the responses.