Motivation needed.

The Roomie and I have decided on authoring a collaborative series of novels. We have started with the framework, complete with mapping out what we want for a total of three novels for the series. And we have gotten as far as writing a rough draft of the first five or so chapters. The problem is, things have gone stagnant. I feel like we are stuck right now. He has been working on his own writing–which is about ready for publishing–and I have been working on a side novel. How do we get the spark back that we had when we started writing?