Putting songs onto it. My mp3 player plugs into the computer fine but when i put songs onto the

Question

My mp3 player plugs into the computer fine but when i put songs onto the player, i go to listen to them and theyre not there.. what can i do?When I load songs on the the player they show up in my media player on my pc but not on the mp3 player itself. it only will play the demos and about 7 other songs.