People don't show up with SS missions because either they don't do it when you're plundering or they have been plundered by someone else. The conveyor who has SS missions have a couple of seconds to show up on the plunders' lists after they arrive in Konaha. I don't know how fast your reflexes are but if every time you see an SS but someone else got it, it must be that you were slower than the other plunderer.