Have you imported a DXF into mastercam then trying to chain it ?
Autocad, for me has problems with splines and arcs, where it breaks them into small line segments. Ends are not trimmed together
Zoom in to a problem endpoint and see how the ends meet
Easiest solution is to open up the chaining tolerance ( allows the endpoint of another entity to have a little "gap" )
There are Chooks for checking for overlaping entities, and also do a "delete duplicate entities" .
On another similar note, try to create curves from solids, not surfaces. Curves from surfaces also create some chaining errors.