That's very strange. I tested his route and it worked for me. Are you getting the option to go to Astaroth's suite after Cain breaks up with you?
Mostly you need to make sure intelligence is your highest stat (by going to the library). Studying the vampires gets you a chance for a scene with Cain, so follow him. After that scene, however, it doesn't matter which fae clan you study. The other things you'll want to do is get your magic high enough that Cain doesn't die, and text him as many times as possible. Be sure to rest (in the lounge) before your stress maxes out (I usually rest when it's at 18/20). It's not necessary to go outside or to the pool at all while on Cain's route. I hope that helps!
Edit: I just remembered--click on the heart to check your affinity with him. When it's at 13/15, you're pretty much guaranteed his best ending.