I am pleased that this has cropped up as i had a bit of a run-in at MAN last week. The handling agent queue-comber (Swissport?) stops people from going to the check-in desks to check-in. The desk are trying to become bag-drop only. I checked in at the kiosk then headed for fast track. They booted me out because of no sticker. Rather than go all the way back to join a growing bag-drop queue I took the normal route.
I mentioned this to the lounge. The only solution is to either check-in on-line, or use a kiosk, then join the bag-drop queue for your sticker, by which time your fast track benefits start evaporating.
I sent a note to KLM (who I was travelling with at the time) but no response.

They will not give the queue comber any stickers; they have to come from the check-in/bag-drop desks, so they say.