The "secret" to sewing displacements is that the corners of the two brushes have to match up perfectly. Not the displacements, the actual brush you created in Hammer. The valve wiki does explain this, but maybe it's not making sense to you. Sometimes the wording doesn't make it easy.
As a test, make two identical cubes in hammer. Line them up next to each other so they're touching. Make displacements, and you can sew them. Now make two more squares, but make one slightly larger than the other. Line them up and you'll notice you can no longer sew the displacements.
Same thing has to happen with your roof brush.. the corners of the face of the brush you clicked on to make the displacement has to match up perfectly with the corners of the walls.
(fine print: the brushes don't actually have to line up perfectly, if a neighboring face is exactly half the size of one face, sewing will work. but don't confuse yourself with that at this point, just figure out the basics first)