Mirror Modifier Weights Not Working I got something rigged with the .L side of the armature and tried applying a mirror modifier in hopes that the weights would mirror and be controlled by the .R side. I've done this before in this project and it worked fine.
This time, however, I got this: 

After some digging, I found out the following:


The selected bone has no control of the mesh whatsoever. The mesh doesn't even have any weights assigned for it.
The part of the mesh that is messed up has weights assigned to other bones that it doesn't need
Trying to add the weights manually doesn't make things better. In fact, it makes it get further away from the bone.
The bones are symmetrical and have the correct names and endings (.L and .R)


I can't seem to find any hint of this problem anywhere. Does anyone know what's going on and how I can fix it?
After a bit of digging when I ran into this issue again, I found out that for whatever reason, the vertex group for that particular bone didn't transfer to the mirrored mesh. As a result, the weights for the mirrored object were there, but the wrong bone was controlling it. In my case, I mirrored to the right side, and that part of the mesh was being controlled by the left side. Here's the solution I found:


Figure out what the missing vertex group(s) are in the vertex group panel. Watch for the one(s) that doesn't have a counterpart for the other side (For example: if there's a hair1.L and no hair.R, you've found one). Select the one for the side that you do have.
Use the grey arrow below the plus and minus buttons to copy the vertex group within the object and lock the original version.
Rename the copy to the bone you want to control it with (For example: hair.L_copy would be renamed to hair.R).
While the bones have control over their intended weights, they will still control the wrong side. Go ahead and fix this in weight paint mode by subtracting from anything that they don't need. Don't forget to unlock any vertex groups that you locked before.