Sounds like you're missing some rivets!
I think ACS lets you order rivets in as small as 0.13 lbs, so shouldn't be more than about $2.28 (plus shipping.) Just keep a list going of misc hardware to buy and throw an order together once you're about to need some parts, to save on repeated shipping costs.
http://www.aircraftspruce.com/catalog/h ... ivets2.phpI've run into issues like this a few times, the hardware lists aren't always as up to date as the plans so the hardware kits from places like ACS/Wicks are sometimes a few parts short. That said, the hardware list on sonex website does call for 170 rivets.
http://www.sonexaircraft.com/documents/ ... e_List.pdfIt's always good to have extra rivets on hand since they're cheap and you're bound to have to drill some of them out during the process. Since the orders are by weight and not by count, however much was in your hardware kit was probably a few ounces short of 170.