Q: have 16gig Nano 5gen can only fit 458 songs on it.  says 12.3 gig open yet will not accept anymore music

I have a 16gig nano that will not take anymore than 458 songs.  bitrate checked etc. songs only take up 6 gig device says 10 gig left open.  will not pleace any music from itunes match i noticed also

how do i get the nano to see the music as on my machine and not on match? could this be the issue?