Why can't I automatically sync music from itunes to iphone?

I have an iphone 4 with OS 5.0.1 and running current iTunes. My music folder on iTunes is up to date with my computer, but when I try to sync to my iPhone it will not sync the new downloads. I get an error message that I don't have enough room to add my library, but the problem is that it is trying to add the whole playlist over again and not just sync. Also, too many files to manually add or use selected artists/albums option. Anyone have any advice on how to fix? Can't find info on iTunes help or in iphone manual. Thanks!