Chapter 7 how do I progress north?

I've gone around and gathered support now it says gather at the highlighted area and proceed north.  It won't let me move north, I have everyone in that post, and it won't let me through the obsidian gate to the south either.  What the hell am I supposed to do?

Also is there some sort of decay on stacks of units?  While I was traveling around with some of my custom created stacks of 4 guys, they all of a suden went down to just 1 guy without any combat at all.  How does this happen?