I'm having the exact same problem and my patience are really starting to run thin with this iOS 7. Things that I class as very simple are struggling to work. What were they doing with all these Beta versions if they can't get simple things right such as moving forward on tracks without there being a delay, listening to a song and then when you unlock the phone the song skips back a few seconds, and now it won't show any of my songs in my recently added folder. Tell a lie, the first time I tried it didn't work, the second time it did. Now I'm up to 6 times of trying after adding more songs and they're still not in the folder, yet they are on my iPhone if I search for them directly.
I'm guessing there is no solution, apart from creating another folder as suggested by Carlanga. But isn't this iOS supposed to make life easier for us rather than having to do something to try and rectify their balls up.