Help with iTunes locating music files

[jayz24rnike]

Hey, I just recently have been importing songs to iTunes using a USB drive. I would put the songs on there and drag them into iTunes. A couple of days later, that exclamation point would appear next to the song and asked me to locate it, so I did. It would work for a couple days, but the same thing would happen next time. I then realized maybe iTunes is looking in the wrong place, so I went to Edit -> Preferences -> Advanced and saw the iTunes music folder location. It is in the right spot, but inside that folder, I have different folders for my iTunes music. Other songs are in folders, why can't it locate the ones I put in there? Like in the iTunes music folder it has a bunch of folders with the artist's name I dragged in there, but also, I already have like 4000 songs and those songs I can't find in there. I don't get it. By the way I'm on Windows. Thanks for your help and time reading this.