Well, I had that situation occur with my wedding.. I had a few gifts whereby cards were lost or left off.. So, in fear of being rude, I narrowed it down just as you did and called a few remaining people. Well, turned out, someone did not bring a gift and felt in their words "caught" .. of course, that was NOT my intention, I simply wanted to thank everyone.. Lesson learned for me... now, IF I am ever in doubt, I just write, thank you so much for having attended "such and such" and for your generosity.. Having you there, made the day even more special.... (anyway, something along those lines)
Now, it might mean you send a gift to someone who didn't bring anything, but I suppose what is worse, not sending a card to someone that may have or sending a card to someone who really didn't bring anything.. I guess you just play it by ear..
oh and that person who supposedly was "caught" :) said, oh we are getting you a gift certificate but it hasn't come yet..... ummmm that was 16 years ago... still haven't seen that gift.. hahahahhaha seriously though..
my best to you and yours