A: Chances are good that most couples experience this, even if only from a few guests. As frustrating as it is, remember what the ultimate goal is: to have as many of your loved ones with you on your wedding day as possible. About a week before the RSVPs are due, check in with missing guests. This is exactly the kind of task you can ask your family and wedding party to help with or to handle for you. Call up guests and let them know that you hadn't heard from them, and would love for them to be there. Remind them that you'll need their answer by the RSVP date, and offer to follow-up if need be. This gives guests a chance to get their act together before it's too late.
If there are still people who don't reply in time, it will be up to you whether or not you (and your caterer) can accommodate them. Despite their lack of consideration, I would advise inclusion whenever possible—while it's annoying that they put you in this position, the goal is still to share your wedding day with your loved ones.