I had this probably too & found, I think, that it was down to not enough memory to sync any new or additional songs across, even though the my Ipod Touch & Itunes was stating that there was still memory left.
Anyhow after trying some of the suggestions out on here & then still being frustrated as the problem didn't seem to resolve itself, I came across an option listed under the 'Ipod Summary' section in Itunes. In here I ticked the box listed under the 'Options' that states "Convert higher bit rate songs to 128 kbps AAC' .
Itunes then readjusted the amount of memory left so there was more & then when I re-synced the Ipod to Itunes, all the songs 'greyed out with a broken dotted circle' started to reload right before my eyes lol!!! There was approximately, 2,400 of these songs so I left my Ipod syncing overnight & in the morning all the 'greyed out' tracks had been synced to Ipod fully & I have all my music as I wanted it... Not sure why this action did the trick but it worked & I'm now a happy little Ipod bunny.
Hope this works for you 🙂