Plat

So I'm retopologizing a head for the first time in 3d coat, and it's going great, except for the symmetry will sometimes break. Heres what I mean: One second the symmetry will be working perfectly, and then, as per my first screenshot, it only shows the geometry that overlaps the x-axis. Disabling symmetry reveals that everything is normal on the one side, but if I toggle it back on the same thing happens. Is too much geometry over the line? No amount of undoing will fix this problem. This has happened twice now. Thanks for your help!