I'm towards the end of the game, and I can't seem to accumulate any more nano catalysts? My counter is constantly showing zero. Aliens still drop them, and I can still pick them up, but it doesn't seem to add to my total. Anyone else experiencing this problem, or is it part of the game? 

I JUST reached that part. I can't say why it won't let you collect more but you shoulda bought everything you needed by this point anyway.

Perhaps because this game is a bit buggy. There were times when I literally couldn't use the melee move so I couldn't stealth kill. Was pretty annoying.