Need help - Specials keep getting blocked

I typically end my combos on a medium, but my SP1s and SP2s are getting blocked way more often then I feel like they should. The biggest problem with this is it leaves me wide open to get countered by the AI. I haven't really noticed this in arenas much, but it happens in virtually all other game modes. It also seems the higher rating of the opponent the more it happens. I have noticed it more with some champs, such as Storm, but I have had it happen with all of my quests champs as well.

Any tips on how to land my specials? Am I launching them too early or too later after my combo?

Thanks!