I added a mirror modifier along the X axis. Then I added some vertices on the mirrored axis. I tried to extrude a vertex along the X axis, but it didn't work. I am able to extrude along the other axes, but not along the X axis. How can I fix this problem?
When clipping is enabled, any geometry exactly along the mirror axis will not be allowed to move. If you disable it, then you can extrude and move center vertices.