I am having the same problem and yes, I am maxxed at 24 too. I have folders according to the genre and I would really like to add a few more, but after adding, even if I put books into the folder, when I click to another folder and then try to click back....the new folder is gone. When I go into the calibre folder on my computer, the new folder is still there, but it won't show up in the main calibre program.
This is driving me a bit crazy so if you have any ideas, please let me know.
Either way, calibre is the best program I have ever used. A huge thank you to all the developers!!!