Milkster,
It may be because the pen is not activated yet, and therefore, you may be limited to 8 patterns at a time. You may have exceeded this limit.
One thing you can try is in Workflow, go to the preferences, then to Behavior->PlanetPress Capture, click on "Manage Documents", then click Search and finally, click "Close Documents". This will force currently open patterns to close, and should free up some patterns on the pen.
Let us know if that works!
Regards,
Raphaël Lalonde Lefebvre