Completing Overseer achievements or any achievement does not result the reward window opening to display your reward. Have to manually key /reward
Are you maxed out on open rewards? You can only have 10 banked. Also, most rewards in ToL (and maybe CoV) are autogranted...you don't get a reward window. For instance, collecitons in ToL now grant both xp and AA, so there's no need to open a window for you to choose.
Or if you have rewards in there that you have not claimed then the window may not populate or if you have it minimized then its popping up but you can't see it.
Not maxxed out on rewards available. If there are any pending to be claimed, they would be separated by their individuals tab, right?
Issuing the /reward command I see the window fully open with its minimize and [x] (close) controls visible. Nothing pending.
Alt + O to open options General Tab 12 down from the top or 2 up from the Load UI button: "Auto Show Rewards" Make sure it is checked/enabled.