I have some sheet bodies, imported from both IGES data, and Parasolid data. When I try to sew any two of the sheets, I'm told they are disjointed, and to try a looser tolerance. I've bumped the tolerance up to .010, and it still won't sew, but when I measure from edge to edge, I get a min 3D distance of 0.000005, same for min local distance. It doesn't seem to matter which two I pick, they won't sew.
maybe you have two or more sheets on the same position. NX shows you the sheet edges, when you press the 'Show Result' button. Then you can see the problem area.