is this shower rarely used? A shower used regularly should never dry out.
Does any of the other water or drains in the home smell like ammonia, or just the shower drain?
The smell is so bad it seems like just the often used shower drain, but i'll have to investigate further. There's a shower, bath and sink all in the upstairs bath. All used often.
So what i'm gathering, is it's probably not the trap. It's probably just a huge mass of female shedding of hair.