I am regularly having problems with symmetry in ZB3 on retopologised meshes.
Here’s an example.
Import the attached OBJ (zipped - exported from ZB3) into ZB3.
Rotate until the top of the head is visible. You’ll see bad sym’ there:
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Use ‘Smart Resym’ in ZB3. Nothing gets fixed.
Now, import the same obj into ZB2 and hit ‘Smart Resym’.
The mesh is corrected to full symmetry.
G.
I haven’t played with 3.1 much yet. But it looks a lot better. The bugs with
zoomed in editing look to be fixed, and that’s great.
But I’m still seeing issues with symmetry. This example (above) is not fixed,
and I’ve already seen other problems where I should have good symmetry
(having done all editing ith symm on) but hitting ‘Smart Resym’ actually
messes up the model.
Next time I’ll try and post the mesh involved.
G.
I would have fixed those points (welded em) before importing, I took a look at the OBJ and those mad non-planar quads on the top of the head are not really needed and definately dont follow the edge flow…see below pic, I hope this helps a bit 
EDIT: just noticed you said “retopologised meshes” but the same thing applies, delete some of the points and make you mesh flow with quads as shown??
Yeah, I know. In my defence it was my first retopo’ experiment. I got a
little too artistic trying to reduce the poly count as I worked my way up to
the top of the head.
That doesn’t change the fact that ZB2 has no problem sorting this, while
ZB3 can’t.
Like I say I’ll keep an eye out for another example. Had one last night,
but didn’t save the tool.
G.
OK. Much simpler example. No retopo’ or anything
Basic ZSphere ‘alien’. modelled with symm turned on.
Polymesh created, symm turned back on and some very basic editing done.
I notice some messed up symm on the back.
Hit ‘Smart Resym’. ZB3.1 reports ‘Full symmetry Found’ But look at it…
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I try ‘Unify’ and ‘Smart Resym’ again. No dice.
Export as OBJ, import to ZB2. Hit ‘Smart Resym’…Bingo! All fixed.
G.
[Edit: Realised I wasn’t clear on one point. I know what caused the
bad symmetry (My first edit stroke without symm’ turned on). What
I don’t like is the fact that 3.X can’t recover from (what has to be?)
a common mistake. G]
Symmetric base mesh as exported to blender for UV mapping, then re-imported.
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As before. I got this fixed by dropping back to ZB2 to resym the import.
G
In 3.1, Smart ReSym works best if you mask the good part of the model.
Yep. Masking one side and clicking smart resym works OK.
Unmasking and using resym again still works with no bad results.
(I guess my export/UV-unwrap/import somehow ‘bent’ the mesh. I’ll
try and figure out where.)
Thanks. I’ll remember the mask thing in future.