Ok, these displacements are all aligned fine, the heights are literally 0.01 hammer units off and still it cant sew them.... Raaaagh///
select the faces in hammer and take another photo. As well as maybe not being the same height or elevation, you need to ensure that all the vertexes of the displacement line up with each other on the X/Y screen before they'll sew.
Only because it's the bottom, i've made it in sections, they all lead down, and the bottom one is raised to connect to the others, is this gonna mean that i has to be the same level as the rest? and then make it go down by painting it?
http://developer.valvesoftware.com/wiki/Displacement#Sewing_displacements It's all about the base brushes, it has nothing to do with the verts themselves. So yes, you make it flat and then move it down.
I just noticed there's a frowny face. The, um, bigger face I saw right away. Apparently the Nobody from Sesame Street found a new home.