Double A,6 and A,7

These hands I have the worst luck with. For example, the times I've doubled an A,7 against a 3 or 4 (in practice and casino play) based on wong 1/2's, I recieve a card that makes my hand much worse! Then I lose. Most of the time if I hadn't doubled I would have won in the first place. If this is really the correct mathmatical way to play why do I usually lose?