Hi gyes! Yesterday while I was sculpting my character I decided to merge 2 subtools- then I made autogroups- and then split groups. In the end I had the same model but I cauldn’t reconstract it to the first level!
Why?
When you merged them, some points may have welded. Or some points of different resolutions got mixed together. So the lower resolution pieces are “blocking” reconstruction.
Reconstruct Subdiv only works if the level above does not have triangles. (Or else weird things may start to happen–I’ve noticed some slightly unusual behavior in ZB3.5r3 when you push reconstruct SubD back to far. Try it on a default cube 4 or 5 times and you’ll see what I mean. 
)
-K