It's the same problem, but I better understand it myself now. What I'm referring to with "highest portion of a height map" and "base" is the "space cube" (for lack of a better definition), which is the cube that surrounds an object, visually showing in a simple way its dimensions. It appears that you can only stitch height maps together if the tops of their cubes are near, rather than if the separated edges of the heightmap themselves are physically close.
The issue that I'm having is that I have to create all of my heightmaps at the top of the valley, stitch them, then flatten them to ground level, all in order for the stitching to remain across heightmaps. I cannot simply create a heightmap at the base of the valley, and stitch it to it's neighbors.
I'm fairly sure it has nothing to do with area extent, the one section that I showed was the highest point at that time, and it worked fine when I stitched it. What I had wanted to do was place it further down, so that I wouldn't have to lower the entire height map in order to have it stitch, and would only need to raise that one high section of mountain.
Thank you for taking the time to help me out.