My 2007 Holden VE Commodore automatic gearbox is slipping gears even after topping up with fluid.
An automatic gearbox relies on fluid pressure as well as volume to make everything work as it should. So, if the level of fluid is correct, you then need to move on to the pump and accumulators involved to ensure that the correct hydraulic pressure is being applied to the bands and clutches that actually apply drive to the gears and physically move the car.
Your problem could be something as simple as a worn pump, but it could also indicate internal wear and tear that requires a complete transmission rebuild. A transmission specialist is your best bet. The good news is that this is a common and well understood transmission, so finding an expert shouldn’t be difficult.