Brep Join - Fail

I am trying to join 4 surfaces together to form a closed brep, but am getting inconsistent results. Sometimes the surfaces join correctly, while sometimes I end up with 1 open brep and 2 trimmed surfaces. In the screenshots I am joining a brep with two surfaces, this brep is comprised of 2 surfaces itself. 

surfaces are not coplanar, but edges definitely match perfectly, as they are all made from each other (ie from brep/surface intersections and split surface).