Why does rig work only on one side of the mesh? I've rigged everything fine, it looks good, it feels good, everything is good. The rig works, but when I try to use the rig, the right side of the rig doesn't work. 

and when I try to use the left side.... 



Everything is good! I still had the mirror modifier so I thought that was the problem, but I've applied it and nothing still works. Can you help me out?
Several conditions have to be fulfilled to make automatic vertex group weight mirroring work:


bone names are correctly formatted, usually suffixed by .L or .R, respectively.
vertex groups for both sides exist on your mesh - these may be empty, but must exist for the mirrored side too. First  LMB select the mesh, &#8679; Shift LMB select the armature > &#9096; CtrlP > Armature Deform > With Empty Groups.
vertex groups must be enabled on the Mirror modifier, but they already are by default
make sure the mirror modifier comes before the armature modifier