I was having some issues with no collision happening after converting a group of actors to a new static mesh.
Specifically, I was trying to use Auto Convex Collision and nothing was happening. I saw collision primitives on the mesh but no option to remove them, too. This was even after creating simple box collision primitives and selecting Remove Collision; the new box primitives would disappear but the primitives that existed on the new static mesh after conversion remained.
The solution for me was to do what TJ said: Details > StaticMeshSettings > CollisionComplexity, set to Project Default. Auto Convex Collision worked after that.