Automatic weight paint failure on only one side of rig Automatic weight paint failure on only one side of rig.
Video: https://app.box.com/s/q7r3yryvmtecm9ljo38f8xst3rjpwk4g
Blend file: https://app.box.com/s/rb1wdvzrdh8qkfu7b2ggv7sjm7xfqc6w
It turns out that the normals were flipped on the left arm. I flipped the normals back, re-attached the mesh to the rig with automatic weights, and it worked perfectly.
I'm still getting used to Blender 2.8. Had no idea the normals were flipped.