Franken 9 Question

I cant pull back the slide all the way. The thing is completely built.. trigger pin, charging handle, etc. However, I cant pull the slide all the way back.. The slide fits onto both front/rear rails. I just cant pull the slide beyond that. I can take the slide down using the takedown button. So.. what am I missing here ?

Ive built numerous glock frames.. so not a newbie..

Thoughts ?