Weight painting queation.

For some unknown reason the weight painting that I’m doing on a mesh won’t mirror it self.
I have the ‘x-mirror’ button on the bottom of the painting panel pressed, and all the armatures are named symetriclly, such as: bicept.R- bicept.L thigh.R - thigh.L ect.
I’ve looked in tutorials and other material, but nothing work. It always paints one side, but not the other.
Anyone know something I don’t, I’d appreciate it.
Thanks

First make sure you have applied scale and rotation (Ctrl A) to both your armature and your mesh in Object Mode.
Then you might want to try the following :

  1. Make sure you have applied the mirror modifier in the modifier stack .
  2. If it is then check to see if your mesh is is symmetrical by turning on the X-axis mirror option in the Buttons Window in the Editing (F9) mode under the Mesh Tools 1 panel . If your mesh is symmetrical then in Edit Mode when you grab a vertex on one side then it’s corresponding one on the other side should move along also . If it doesn’t then delete one half and reapply the mirror modifier again and re check .

Hope this helps (and I hope that you don’t have any shape keys on your mesh) .