Those little ******* square shaped rubbers, I lost one of them while cleaning mine, it got dropped into the air duct!
Then I devised a genius solution for it: I cut a small ring from a small syringe (used to inject insulin usually, very thin), wrapped some surgical tape over it and pushed it inside that square space to give leverage. The vent is perfectly on its place, though I feel it to be slightly "less" sticky in motion as compared to the other one.