You'll need to provide examples. Does your firm not allow over night positions, or are you shorting REG SHO stocks without locates or pre-borrows. Many small prop firms don't have the capital for your leverage over night.
MON is not hard to borrow. What was their reason? There are too many reasons for me to guess. When you're within a JBO/Prop firm, they can make their own rules and you have to follow them.