Having just spent hours on this problem I’m posting here.
I have a simple square made up of Instanced Static Meshes, I’m then trying to run around on them and it just won’t move even though seemingly everything is working. I can see the NavMesh is not generating anything on them, and it will if I put some other static mesh in there, but I don’t know why.
Having beat my head against this one and tried everything ever suggested on the subject I did the obviously-not-going-to-work-but-clutching-at-straws-thing of restarting UE4. It now works.