Maya not mirroring weight paint perfectly

[Commodore]

How come it's doing this?

http://media.newschoolers.com/uploads/images/17/00/48/52/52/485252.png

[daleovl]

Try setting both your influence association in the tool to one to one and then surface association to closest point. make sure that you are mirroring across the correct axis as well. Maya tends to do that when it can't tell what joint to give influence to, this is usually due to your settings for the tool

[xillyriax]

Also make sure that your center line of vets are actually snapped to a gridline or perfectly vertical. I've had Maya spit out errors if those verts aren't perfectly straight.