Issues with Rekordbox playlists

Hello, I'm wondering if anyone can help me.

I've just upgraded to a XDJs and I'm using Rekordbox for the first time. After adding my first set of songs to the collection it won't let me add them to any of the playlists I've already created. Would there be a reason for this, it's probably (fingers crossed anyway) an easy fix but it's got me stumped.

Any help would be greatly appreciated

Adam