Rule number one of whetstone sharpening, make sure your stones are dead flat. While sharpening you will almost certainly "dish" your stone, by this I mean the centre part will erode quicker than the ends. It is paramount that you level the stone before sharpening as you won't be able to keep a constant angle if the stone is dished.
"Flatten enough stones and even your flattening stone will need a flattening. Just rub two of them together and Bob’s your uncle you are back in business."