I have had the same problem but it seems to be easier to get screenshots accepted when you have the game on windowed mode, put the game window on the left with the completed task showing well and on the right put your browser with your gamehag profile opened. Hope this could help you out :)
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99% of the time I have to repeatedly send my screenshot to misty because she rejects them constantly though they clearly show the task being completed with my name on it. Im just confused how sometimes she can accept it after the hundredth time and most times she just wont accept at all?