Congrats on the litter! Not finding slugs if she'd already finished giving birth doesn't mean that there weren't any. Female rainbow boas will often eat them and I've even seen them eat them while they are still dropping babies out the back end! As long as your caging is set up for the babies to thrive in, I think you'll see a change in increased feeding within a week or two after their first shed. Good luck and enjoy!

Charlie