I can't get my arm back 180 degrees either. It bums me out because I definitely feel that if I was able to, my velocity would go up quite a bit.
I'm certainly not qualified to say this, but I think that it's because I haven't been playing baseball all my life (only have 3 years under my belt), let alone throwing.
I think that that kind of shoulder flexibility ultimately comes with time.
Opinions? Can proper mechanics really immediately help a pitcher get their throwing arm back 180 degrees?