When that option is on, you should see a "View Filter Reasons" option on the menu on Select Music. (which appears when you press Select)
Choose that option and it'll show you a list of all the songs and the reason filter that removed them. It will also save the list to a file.
song_too_long means the song was over the time remaining for the credit.
song_censored means the song was censored.
song_fits_rating_cap means that the song didn't have any charts easy enough for the rating cap. (rating cap is a per-profile option for hiding songs that are expert only. If a song doesn't have a chart under the cap, it doesn't show. Set the cap to -1 to disable)
< maxvg1> shakesoda: then why do i still play lol
<@shakesoda> because you're an ITG player. And thus, a masochist
<@shakesoda> Kyzentun: I think you might need to put down the meshes for a bit