Feeling Frustrated

I’ve got like half of a horror story in my head and it is driving me crazy that I don’t have the other half.
The problem isn’t plotting because I haven’t progressed to plotting on this idea yet. I know I want to write a werewolf story. I want to use werewolf mythology older than Hollywood’s. That is I do not want to use the tropes that were created, wonderfully I might add, for the film The Wolf Man.
I know I want to use teenagers and high-school and I might even want to make it a period piece set in the 1970’s, but I am still missing the undefined element that makes me go, Bang! I’ve got a story.
All sorts of elements and scenes have formed in my head and rough character outlines, but not everything I need.

It’s so terribly frustrating to be this close and not have what I need. It’s like when you;re trying to remember a name or title and it just will not come out of your head.