Why can't I mirror my vertices?

Can you post a screenshot of when you try and move the Vertices on one side? (Full UI, please)

If it is not one of these two things:

Steps:

1. Make sure the origin is in the center

2. Delete the other half of vertices. 

3. Make sure the symmetry is on the Y axis. Look on the viewport, Suzanne is facing on the  X axis however her ears facing  on the Y axis.

So after trying to get my screen capture to work(and failing) I have decided that instead of using mirror for both sides, to instead just select all the vertices for the switch and scaling along the x-axis. I appreciate you guys wanting to help.

You can use Blender itself for the screenshot:

So this is the image. As you can see I moved the right side but the left side didn't move, even though I had the X-axis mirror enabled above the viewpoint rotator.

That is weird. There is one thing I found: when you have Topology Mirror enabled, that might cause this:

(Don't ask me what that is for...)

Otherwise I have no idea what could be causing it.

As long as the Body is symmetricall, you could also use a Mirror Modifier in stead, like euv mentions above.

Yeah, it must be a bug or something because even if I don't have topology mirror checked it still doesn't mirror properly. 

Did you try this on just the default Cube? (New file.)

Are you using any third party Addons? Try it with: File > Defaults > Load Factory Settings.

Also make sure that your graphics driver is up to date.

ffrankfriesen I don't think there's a bug. There's no loop cut in the center of your object. That's why Blender doesn't have a clue what you want to do.

Here's my example you should do with the mirror.

Another way to mirror an object:
1. Make sure its transform pivot point set Median point (default) to 3d cursor (align with the origin).
2. Turn on the mirror modifier and choose the desired axis.
3. Go to edit mode and press A to select all. Move the object in the same position as the axis portrayed on the mirror modifier.

euv I agree with you that it's probably not a bug, but there seems to be a slight misunderstanding here; it is not the Mirror Modifier that is giving us sleepless nights, but the Edit Mode Symmetry/Mirror (which is used in this tutorial).

Hey all,

It looks to me as only 2 things will cause this issue,

1, The origin is not in the centre of the object, on the mirror axis, in this case the x axis.

2, The geometry is not symmetrical on the x axis. If you moved the verts, even a fraction then turned on x mirror, it will not work.

To solve this I suggest add an edge loop in the centre, delete one side of the mesh, add a mirror modifier, enable clipping on the modifier, move the edge loop you added in the x direction until they stick in the centre, apply the mirror modifier,  delete the edge loop and continue.

Excellent points @adrian2301 !

Although it looks as if the Mesh is symmetrical and the Origin is in the center, there might be something slightly off...

Sounds very likely, actually.

Alright. Thanks for all your responses! I will keep this in mind for my next project!

I had the same problem and found that it was the Vertex in the X axes that was not symmetrical. thankful for frankfriesen Q&A.

I have the same problem. I centred out the body to 0 on X. And transformed from 3d cursor which is set at 0 0 0.

As you can see some edges mirror but most don't.

https://app.screencast.com/NNgp1YjYVffxI

I don't need to fix this but I'd like to understand what's gone wrong if I can

Still can't seem to add images direct to posts

This is my workaround

1. Move on the x to about the right position
2. Make a mental note of the x value.
3. Undo and then set the X to nearest integer
4. Select the other side vertices and set that x to negative of the previous integer

I've now worked it out. The trouble I had was sometimes that button isn't visible.