I'm on mission 30 and I've already fultoned a few S-Rank soldiers out; however, in my roster, I can't find them at all. Am I missing something? My iDroid is up to date on upgrades and so is my fulton, so I have no idea why they are vanishing and I can't seem to find a solution searching with Google.
As no additional info are provided, this is the likeliest answer:
The soldier you fultoned were shown as S rank, but were Boaster. Which mean while you see them as S rank on the field, they really are A rank, or even B rank. (rumors say they could even be as worse as C rank). When they get in the Brig, their real stats are shown, and as such don t appear as S rank anymore.
People don't actually arrive at Mother Base until you hit a checkpoint. When they do, you'll get an "Extraction Flight Arrived at Mother Base message", and anything you picked up or Fultoned will enter Mother Base's inventory. You can trigger a checkpoint by leaving the map area (though it's not clear where the triggers are) or by continuing the mission until you hit a milestone that gives you a checkpoint.