I can't say for sure
but I know ipod has a proprietary mp3 format, and
perhaps the frequency and bitrate you saved at are not compatible.
if you save at a non-standard rate something other than
a full wave or half wave or qtr wave 44.100 - 22.050 - 11025 @ 256 or 128 or 64 or 32
then maybe it just can't "read it " properly
that's my best guess....maybe someone else has a better answer...