Interface problem

Tomislav

I am meshing a heat exchanger. I have an interface between air and a solid fin. In gambit I define this boundary as interface. In the first try Fluent automatically recognized it, and generated a shadow face. Now, with similar meshes, when I try to do it, Fluent does not recognize the interface, so I have to define it manually. However, when I solve it, the solution diverges or I obtain floating point errors. I supose the reson is the interface. What I am doing wrong? How can I make Fluent recognize automatically the interface and generate the shaddow face?