Have you got the prison? If yes, then maybe your strength or endurance stat is too low to carry him? try raising them up by a certain amount until you manage to get him, and the Reptile Lure adds 25 to the counter each time you use one on him, and it goes down by one after each round you and the enemies do, if it is less than 100, then it is a chance to capture him based on how close that is to 0.