Ahh.. this might be it: on the copper side of the board, there are tiny little resistors near each MOSFET. One side of each resistor connects to the Gate, but I cannot see where the other side connects.

Have a look and figure that out for us, will ya?

If they connect to what used to be the big fat ground trace, then that's the problem. You'll need to somehow restore a true ground to that side of each of the resistors.