SIMs recruit differently from humans. They start with interest in a certain number of recruits, and add a certain amount of interest each cycle with no limit. The only time they stop is when a human pushes them off the recruit, so if you've got a recruit yellow with a SIM on them, the SIM will eventually get them if you don't do enough to turn the recruit green. The longer a SIM is on a recruit, the more interest it'll build up, so it's best to jump on SIM recruits fast and hard to minimize the cost.