A few years ago this might have been the case because the character model you wanted to use was of a different gender (male ≠ female). In fact, I think it's currently possible, and this limitation has been removed or no longer exists in current versions of BR and HF Patch.
It's probably a folder within another folder, and you're not selecting the character itself at any point. That's what I can understand as a possible cause of the problem. 🚬
What version of the game do you have, and what version of CharaStudio are you using? You're probably missing a plugin.