1) If you have custom categories and haven’t exported user data recently, do that first, so you can restore the categories that are still working.
2) Close DS and uninstall CMS.
3) Reinstall CMS by running the installer in C:\Program Files (x86)\DAZ 3D\DAZ3DIM1\cms (if you have DIM installed) or by reinstalling DS. That will give you the latest CMS, which will reduce the chances of the problem recurring.
4) Search for the file ContentDB.ind—if you let CMS install to the default locations it will probably be in Program Files (x86) or ProgramData. I would search your entire hard drive with “Hidden and System Files” visible, in case you have more than one set of databases, which could also cause problems. That file is the index for the database.
5) Stop CMS and delete ContentDB.ind, then start CMS and launch DS. Let DS rebuild the index—it might take a minute or two.
6) Scan for new content and categorize it.
7) Close and restart DS, and see if the categorized items are staying. If they are, you should be good to go.
8) If newly-categorized items are still disappearing, close DS, stop CMS, and delete all the files in the databases folder. Restart CMS, launch DS, and re-import metadata to get your user data back. It can take a long time, so don’t panic if it seems to be taking a long time and the progress bar isn’t moving.