check the following:

Normals - The normals of both objects may be opposite each other. Use the edit polygons > normals > commands to fix.

Merged Vertices - check to see if the vertices in that area from the two objects have been merged together.

Doubled geometry - check to see if you have doubled faces or edges.

I've got a mirroring and attaching video on my site that may help with this. Not necessarily the mirroring part, but the attaching.