I can't convert higher bit rate songs to 128 kps?

I have at that option checked ever since I got my iPhone 4S. And just in the last couple of months some of the songs I add into iTunes will not convert and give me an error upon syncing my iPhone. Somehow I have figured out some ways around it and got it to work again. But now I removed all my music hoping that if I did that and sync again it would be able to convert. But now iTunes won't convert any of my song files which really makes me mad when it has converted them MANY times in the past and worked perfectly so now I have no songs on my iPhone and really want them back on there. I want that option checked to save memory on my deivce.

Thank you for any help. This has been such a pain over the last several months. Usuall I am pretty good at solving problems on my own. This is my last resort.

I am running Windows 7 running iTunes 10.6.1.7.