I'm using Nokia Lumia 520. The problem is when I upload songs to my memory card, the phone reads the sd card but the music contents won't reflect on the music+video icon. It only allows me to play music saved directly to phone. I've tried switching the music preference of storage on Settings from phone to sd card and vice versa but it doesn't work
After being infuriated by this same issue for an evening/next day, I started trying anything I could think of. For some reason by simply making a new folder in the SD card, which I called songs, I can now play all of my music from there. The problem seems to be adding files to the music folder which the phone doesnt recognise as being music files so it deems them unplayable. I really hope this works for everyone else with this problem as solving this has made me unbelievably glad to have my music working.