It looks to me like you are not getting good snap because you lack good forearm/wrist tension.
You're not getting your thumb opened up enough on your reach back, which means your muscles aren't relaxed and stretched out on your reach back. You're not resisting the inward bend on the way to the power zone, which prevents good loading of the forearm muscles/wrist, and because of those things, you're not getting good tendon bounce or thumb extension at the apex of your shot, which is what ejects the disc from your hand.
You can watch almost any pro and see them open up their wrist a little bit right before going into their shot (see video of Nikko for a good example). When you do the Nikko pre-shot routine, you feel a little stretch in the forearm all the way through the base of your thumb, and that's the feeling you want on your reach back. Those are the muscles that produce snap, and you aren't loading them up, which is what's preventing you from getting explosive throws. You'll get a real heavy feeling in your forearm when you really snap the **** out of a disc.