Bioshock: Get the Little Sisters to appear from the air-vents?

I’m playing through Bioshock (Bioshock Collection) and i’m almost through Neptunes Bounty, i’m lacking two Little Sisters though. One in The Fighting Mccdonaghs Tavern and one on the second floor överlooking The Port. There are two Big Daddies patroling here knocking on all the ”air vents” but no little sisters appear from them. I’ve tried luring them to other vents using the ”hypnotise Big Daddies plasmid” but i just can’t seem to lure them out. What am i doing wrong? Just FYI, i’ve also tried not getting spotted by the Big Daddies while they do their patrol route but that doesn’t seem to help either.

Try killing each Big Daddy and then wait for them to respawn at which time it should go to a vent and grab a sis.