tracks not playable on ipod

I recorded my band using a sony PCMD50 digital recorder. They were recorded as 96 khz 16 bit wav files, which were than imported into itunes and converted to apple lossless.

After converting to apple lossless, I tried importing them into my ipod, and got the message " "filename " was not copied to the ipod because it cannot be played on this ipod"

Does anyone have any idea whats going on ? This is 160 gig ipod classic..