ipod help

I downloaded more music but when it is time to snyc it tells me there is no room only 45 MB left.But my Ipod has 869 MB left.I have a ipod Nano 7.3 GB.I currently have 815 songs on it and I have had 1800 songs on it.I think it hold up to 2500 song.
Anyone have any idea what is going on?How can I fix this?

Thanks,Jim