After having a lot of problems snapping to AEC walls and not always been able to duplicate the problem, I think I finally got it. is the "0" Layer. for some reason when layer "0" is off or if the layer is not in use, you cant snap to the outside of an AEC wall. Even if you have it on and in use, if you have an XREF drawing that i snot using the "0" layer or the layer is no in use, you have to go to the XREF file and draw something in the "0" layer. that will allow you to snap.
So bottom line, draw something on all your files in the "0" Layer and keep it ON. that will save you big headaches.
One more thing if you turn off the current layer in the xref you wont be able to snap to XREF AEC objects. so go to your XREF file set your current layer to "0" and don't turn it off.
