I have two machines.
On one of them I can't get ZSnapping to work.
another works as documented(most of the time).
on that problematic machine zsnapping only occurs if tolerance is enabled and set to a proper value. I retrieve feature(point) from file geodatabase(gdb), take its xy coordinates, do a bare IPoint from them(only a call to PutCoords) and then do a FullPointSnap() on it. The method returns TRUE, but Z coordinate stays NaN. Setting spatial ref system or ZAware=true doesn't help.
it doesn't even do ZSnapping in ArcMap UI after I manually start edit session and try to place a line vertex. But works ok once ZTolerance is enabled and set.
ArcGIS 10.2.2 automated through .net4