In order to gain and sustain an edge on any blade; the flat or back of the chisel needs to be flat.
Often, due to the manufacturing process, the back of the chisel or plane blade can have a slight hollow directly behind the blade bevel. Prior to sharpening, check to ensure the back is flat. So keeping the back of the chisel flat on the de-burring plate and using a simple figure of 8 action Gently move the blade back over the plate. At the very least you want to achieve a continuous flatting mark across the width directly behind the chisel tip. If you have difficulty seeing the flatting mark. use a wide tipped felt marker. Flat again and that will give an instant indicator of progress.
However if the blade back shows considerable signs of distortion. Flatting the back on a larger traditional oil-stone or larger diamond plate will accelerate the process.