There are two types of throws from Shihonage. You are currently doing the throwaway version. Master that then move onto the throw directly down version. Both are valid and used for different situations. I was first taught the throwaway.
Yeah, that was what I was wondering reading the original poster. Is he saying he's currently throwing out and wants to throw down, or that he's trying to throw more outward than downward but not succeeding? I think the former but it's not really clear to me from the wording of the question.
Personally I was taught the outward version first, and it's the one I generally always aim for if I'm not trying to pin.