After you have the card returned and set the deck down, with the looped threaded through the deck, there will be more tension on one side than another. You can re-calibrate the tension by bringing your hands together over top of the deck in a praying position (as your lightly rub your hands together) and then returning them to the palms down position for starting the animation. This will even out the tension.
Two other tips: 1. When the card is returned try to have it returned somewhere in the top quarter of the deck so the loop has less cards it needs to move. 2. Don't rush the animation, but don't go too slow either. Setting the tension quick, or prolonging it, will cause the loop to snap.
Hope that helps.