Some songs simply will not down-convert.

I'm loading a new iPod 5g with music, downconverting to 256 kps, but some songs will not convert. When I "Get Info" on the iPod, they are still at 320. It appears to affect random songs, and even some songs from the same album will convert but others won't. I have deleted the songs from the iPod and tried again, I've unplugged the device from the computer, turned off iTunes and the iPod and reconnected, made sure the source folders are not set to "Read Only". Any help would be greatly appreciated.

(I know audiophiles are going to cringe, but I'm doing it to fit more songs. I can't fit everything i own even at 192 kps, and the conversion from 320 to 256 will allow me to hopefully put several hundred or so more songs on the iPod).