I have about 6150 songs, and I only just noticed that a lot of my recently added songs are not showing up in the all songs list. They don't show up under the respective albums either, and there seems to be no pattern for showing up in Artists. However, they show up with no issues in the "Recently Added" playlist, or other playlists.
As a sidenote, on the "Now Playing" screen underneath the artist name on the top left corner where the song number/count is shown, the count is correct (i.e. it will increase when I add new songs), they just show up.
Noticed this on Kitkat and Lollipop both.