Hold it up to the stock one and look through it. Align the key ways and see what the problem is. If the key way is too narrow then send it back. If the woodruff key hammerred the slot bigger in the stock one then you may need to replace the woodruff key.
Sometimes during removal the woodruff key will tilt in it's slot. It is a crescent shaped key way. It may need to be tapped in and straightened. I would not file it, but I have rubbed them on 600-800 grit sandpaper on a flat surface to clean them up a bit.