Hello, when I use sims4studio to change a CC so that both genders can use it. When I view it in CAS in game on some of them the midsection of the sim is missing, is there any way to fix the CC so that it works for both, or is this just because the CC was created in blender and there is nothing I can do?

If the item was a top or bottom, body parts are included on such items and the seams likely don't line up between different bodies. They cannot simply be toggled to be compatible with all frames.
The arms / torso / legs included in the clothing need to be replaced.

Aw jeez, this sounds like it would not be an easy fix. How would I go about replacing the arms/torso/legs if I were to try?

It's a little complicated to learn, but easy to master. I'm personally more familiar with TS3, so I won't go into too much detail just in case I mix things up and make it more confusing. I'd recommend checking out the Sims4Studio forums for their tutorials and discussions. General clothing creation tutorials will have sections on how to add the clothing to the body pieces, so you can skip there.

You can get a basic idea of how it works by exporting the mesh of the CC, or any similar EA clothes you might want to use as examples. Open in Blender and you'll be able to see which of the mesh pieces are body parts that need to be swapped out.
S4S is quite advanced these days, there may even be easier methods to do it now.

Thank you for all your help, I will look into it.