Hi
In Blender, or in the DBPRO? In DBPRO objects are displayed without back faces when [Set Object Cull] is set to 1; by default. Setting it two 0 will force the engine to draw both sides; which drops performance at high levels of polygon count.
You can also flip the direction of certain face normals using the Flip Normals button (CTRL F, then 1) in the edit mode toolbar in Blender. The Re-calculate Normals (CTRL N) button, often works well for when you fill holes (F) or attach new faces to meshes.
You can see what way normals are facing in edit mode by showing the transform panel (CTRL N), Mesh Display > Normals > Faces.