Why my iPhone 5 shows partially my music even if I synchronize all of it using iTunes?

I have synchronized all my music with my iPhone 5 but after 2-4 days it seems that part of it is gone, however when my iPhone is connected to iTunes and I go under Music section I can see that all my music is there. I don't know what happened but this is the 3rd time I have to erase all my music and synchronize it again in order to see it complete. Can any one help me?