Ipod wont sync new purchases from itunes,but lets me sync songs that have been imported from cd ,

Have an issue where my ipod wont let me sync some new songs that i purchased from itunes(song shows up in my ipod,but stays greyed out,with a circle of dots ,to the left hand side).

When i import a new cd ,the songs transfer over to ipod straight away with no issues.

anybody help?

Thanks