Sometimes photomerge can't figure out where everything goes - so it breaks the panorama into sections that it knows go together.
I would be very surprised if that were the case. In earlier versions they had plug-ins to correct problems like this. Again it looks like they have a maximum canvas size and if the picture exceeds that size, then the panorama is broken down into separate pieces.
I've been trouble shooting and found the problem. It is with the maps pictures. Something was different between how the maps were compiled. I'm recompiling all of the maps. Photoshop is is doing an amazing job of stitching them together. The file is going to be very big. I hope I can get it printed. This is a screen clipping of my progress so far.
Thanks for checking back.. Glad you were able to figure out the problem!
Just a quick P.S. If the curve in the panorama is unacceptable, you can use the Adaptive Wide Angle filter to make it straight. If you aren't sure how to use it, a quick Google search should get you going.