It is inconsiderate, but unfortunately common, for guests to fail to RSVP.
Anyone who receives an invitation has an important obligation to reply as soon as possible. And yet so many don't. Some forget; others procrastinate and then feel guilty, so they delay even longer. To many a host on the non-receiving end of an RSVP, it seems as if an invitee is simply waiting for something “better” to possibly come along. One of the sad parts about the demise of the RSVP is that relationships often suffer due to hosts’ resultant hurt feelings and frustration. It is perfectly polite, however, for hosts to call friends to ask if they plan to attend. In fact, if you want an accurate headcount, you have no choice but to call those who haven't responded and ask whether they plan to come to your event or celebration.
Yes, it's an awkward conversation. Be friendly, not accusatory. Say something like this:
"Hi Janet, It's Marion. I'm calling to make sure you received the invitation to Uncle Jimmy's 80th birthday party, as I haven't heard from you. I hope you and Alex can attend. I need to give a final number to the caterer on Tuesday, so please call me back and let me know if you are able to be there. Thanks!"