I used to get this:
But lately, this hasn't been appearing for me. I know I have been gaining reputation, but It's not showing the green icon for it. I don't think I really clicked anything different, so why did it disappear?
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I think it's because You lost 500 points.
The achievements shows your total rep, i.e -500 + whatever you gained, since it is still under zero it will not be shown.
But you can still click the achievements icon in order to solve the problem(the -500 will not be counted anymore)