So, about the skater girl...
Her friends all stood around while this guy spit an alien into her?
In the other situations the infested and their target are alone, so because the target doesn't seem to be aware there's a creature climbing into them for whatever reason, the event goes unnoticed.
But with the skaters they were all standing there while this infested guy attacked the girl, and it takes a good 5-10 seconds for that creature to transfer.