I figured that must be it, but it didn’t work on the dress I made, but worked fine on a tessellated test cube. After some more research I found that in order for it to work, it must be perfectly symmetrical. During the initial construction I did use some proportional editing and can only guess that this may have messed up the symmetry, although I never noticed anything wrong.
In future I’ll just do any tweaking it needs on one side, delete the other and do another mirror modifier.
Just for interest, Hexagon also won’t do symmetry if the model is not symmetrical, but is not all that fussy about perfection. What it does when you choose to model symmetrically is give you a symmetry plane. If the plane doesn’t show up, you know it won’t work and the model is not symmetrical. Be nice if blender had a similar early warning system.
Anyhow, thanks for the advice