Zimmerman Trial - Why do cops/analysts not know what actually happened?

Why does forensic information NOT explain where both GZ and TM were when Z pulled the trigger? Between blood spatter analysis, the trajectory of the bullet, gunshot residue, etc., one would think law enforcement would have at least a pretty decent idea of what actually happened, who was on top, how far away the gun was when fired, etc. Or has this been covered and I just missed it?

Before anyone accuses me of watching too much tv, I don't watch any crime scene dramas. None. Nada. Zilch. I read a lot and I've seen a few movies that touch on crime scene analysis, but I never watch any tv shows about the subject. From what I've heard, those shows are bullshit and not much based on reality.

So what gives? From what I have read about the subject, the detectives, the coroner and the crime scene analysts can usually come up with at least a good working theory about what happened. Why is there no conclusive idea from law enforcement about how it went down?

Did the cops do a shitty job of the investigation? White guy kills black teenager . . . case closed?