Thank you for taking the time to do this, it's a honor.
Everyone writes a bad song, verse, chorus, every once and a while. I'm interested to know how you go about working with the artist when you think a part of a song or the song itself could be better?
Good question... if the song is bad, do another one or get a re write. You have to be brave enough to tell them. If you're a musician or writer, help them out. (I'd check publishing arrangements before you get too deep into it ) If part of song is weak it may need arrangement in drums or bass. Sometimes a new imaginative bass line in a weak verse or bridge can really lift things. Do it in rehearsal pre production, not in studio.
Cheers
JL