No forced sub plots
One of the scripts I read last week forced a flimsy sub plot. I was the 2nd to read it, the person before me found the sub plot more believable than I did, so , it's up for a re read. At best, it could be made into a MOW (Movie of the Week).
What I learned from reading it:
If you have to force the sub plot, the audience/reader will feel that and come out of the story and into critiquing the writing.