John,
Another thought; is it just this one board with the issue or do multiple boards have it? It might a manufacturing issue with one board such as a bad via or plate through,
Note that the thickness of the copper is set by the PCB stock used to manufacture the boards, asking for heavier copper will add cost; you specify this when you order the boards; I don't believe you can ask for thicker copper on selected traces.
I would think what needs to be done is the design of the board needs to have the width defined a little wider for the power bus traces; these are already wider on the boards when I examine the actual boards so I'm assuming it's a setting in the design software. I would guess this is defines in the design rules.
Andrew may know better so hopefully he can weigh in.
-N