When snapping the plastic cover on you have to start at the back. Remember the notches?
Make sure you have the button in place and facing the right way.
Then I just hold it like this and snap it back together. If its not all lined up in the back and that metal piece I showed upside down isn't in correctly it wont go back together.
If it is all correct the sides of the plastic will be flush with the metal buckle part.
I hope this helps somebody.