Dense vegetation is always a big trouble. In windy day it's even more changeable than water surface.
All tree leaves looks the same. And the same tree looks so much different when photographed from above or pictured at some angle, as on image periphery. That makes PhotoScan duty nearly impossible.
We too often have gaps on forestry areas.
Sometimes it helps to increase image overlap and exclude oblique parts of photographs. Sometimes not. Fortunately we don't have big forests and may get photo aligned thanks to some roads and rivers crossing the vegetation areas.
Starting from version 0.8.5 there is an option of using markers as user set tie-points. This works if you run "Align selected photos" from context menu. However it needs a lot of handwork, 'cos you need to find and mark many fixed (i.e. not moving like tree heads under the wind) points.
To be honest, I tried this approach on dense forest only once and didn't succeed.