Why are some songs not showing up in my playlist on my iPhone 4s?

I'm trying to organize my music so I made playlists. I put all the songs I want in the playlist on iTunes, then I go to sync my phone and about half the songs are missing? I tried everything just doesn't make any sense

Also I tried to put the songs in the playlist on my phone itself then I sync it with iTunes and just takes the song right back off? This is frustrating

anyone have an answer to this problem lemme know please

I have the same problem and I'm trying to figure out what to do.