How do you get music from the MP3 Rocket PRO to your Ipod?

I am using the MP3 Rocket PRO app to download my music and it was supposed to sync the downloaded songs to my Ipod. I have done this before (at least three times) and it worked perfectly fine, but this time, it only downloaded four songs (out of the twenty). Does anybody know how to get ALL of my songs downloaded to my Ipod? I would really appreciate some help!