Father-in-law Mike and I spent an hour the other day trying to fix a cabinet door whose hinges were bent and broken. Unfortunately, the cabinet door was installed in such a finicky way that we couldn't simply replace the hinges with hinges from the hardware store, and it took us a good deal of time just to understand what we were up against. There's no room for the cabinet doors to swing out, so they were installed with fancy hinges that swing the doors away and
out, and the replacement hinges that Dad has don't swing them enough away.
Either they were designed by an architect who didn't care how much trouble he was causing or they were built by a carpenter who didn't get around to the cabinet door problem until he'd already built all the cabinets, and had to go find fancy hinges that would fix the problem he'd created.
Every professional writer gets this call: "We have a script we're interested in, but it was written by the producer's niece, and she's never written a script before, but we're real excited by the project. Could you do a polish on it?"
I then have to explain that you can rarely do a polish on a script written by an amateur. You almost always have to take the damn thing apart and put it back together again before you can even start polishing it. It is usually just about as much work as if the producer had simply taken the idea to you and commissioned a script in the first place. Of course you're getting paid 40% less -- a page one rewrite is still a "rewrite" not a commissioned script.
When an amateur writes a script, it is often missing some of the following: a hero (or anti-hero) we care about; an opportunity, problem or goal for the hero; obstacles and/or an antagonist; jeopardy; stakes. I read a script not that long ago where I couldn't tell who the central character was until page 80.
There may not be characters. The characters may not have their own voices.
In other words, the elements of the story -- the structure of the story -- isn't there. And I have to explain why I can't polish something and give it story elements. I have to rip it up and redo it.
A dialog polish is like slapping paint on a cabinet or dresser. It makes it look better. But it doesn't make it work better if it's not working right. If it's not working right, you have to take the thing apart, usually all the way, and recut or replace some or all of the pieces, and then put the whole thing back together again before you paint.
Likewise, if an aspiring writer asks if I'll write with them, then the answer is almost always "no." I'll consult, or I'll do the writing myself, but I can't co-write with someone who's not a pro writer.
This would be like a carpenter building a dresser with me. I really can't help her much. If I cut something, it won't be to the same tolerances as the carpenter's work; the pieces won't fit right. If I put a piece in place, I'll glue it in the wrong position and throw everything off. The carpenter can usefully do the work herself, or she can sip coffee while telling me how to do it right. But there's no "co-carpenting."
I find that once you start using carpentry metaphors talking to producers, they start to understand better why you can't "polish" an amateurish script, or "co-write" with an amateur. Try using this metaphor yourself, let me know how it works out.