Hi Matt,
I think I failed to mention that there is a mirror modifier on the mesh. This means that only the left side (his left side) of the character is real geometry and all the verts on his right are actually generated. This means that both sides are going to be exactly the same – but you can’t paint the right side because those verts aren’t actually “real”.
The other issue you might be having is a common weight painting where you are splashing weights on the front facing verts but some of the others are being occluded based on the view angle. They then end up tucked inside the mesh and it becomes impossible to get your paint brush in there (if you know what I mean).
This is just an experience thing. You will learn how to angle your view when you work so don’t leave some verts behind. Practise and experience will make this automatic for you. (You’re already on your way)
