Issues with one or more customs’ DTA files can often cause songs to disappear from the game. Unless others have knowledge of specific issues with these songs, it will help to get familiar with how the DTA files look in customs that aren’t causing you grief, and compare that to the problematic customs’ DTA files.
You can use C3 CON Tools to look at and edit the DTA file within a custom. Just look for the “Quick DTA Editor” button. Note that DTA changes require another cache wipe to take effect.
You are about to leave Rhythm Gaming World and access the external songs DB. While no song or file sold for commercially available videogames is hosted there and only fan generated content is allowed, Rhythm Gaming World has no direct control over the database and no files are hosted here. Alright, we had to made you aware of this, now you can visit the database!