What would make one side of a v12 stop working right?

What would make one side of a v12 stop working right?
Lots of things could happen. Due to new laws about ethanol, ethanol is going into cars that aren't designed for it and over time it dissolves stuff in the tank and deposits it in the fuel injectors. You can replace the injector or simply put some fuel injector cleaner from most auto part stores. Spark plugs need to be replaced and there needs to be a precise gap for each and every car. At high temperatures and speeds (typically common uses for a v12 engine) the pistons and cylinders are likely to wear down. Engines need to be lubricated and cleaned. You can get engine cleaner from your auto part store in the oil section. I recommend Gunk brand from Oriely Auto Parts.

Answer 2.
It is unusual for just one bank of a V engine not work properly. It is possible, (assuming it is an overhead cam engine), that the cam timing has slipped a tooth and is mis-timed. Check the valve timing.

Answer 3.
Also they make equipment that plugs into the plug under the steering wheel and they can tell you literally everything that is happening in the computer system in your car. I recommend that you take the car to the dealer and check the system instead of buying an industrial strength $1000.00 piece of equipment.