Are you certain you're getting a full charge on the battery? When I first bought my Aerobird Challenger, it came with the 12v car charger only. I'd charge the battery with the car charger, but the charger would "false peak" and turn off before the battery was truly charged. I could get my motor to spin up for about 5 seconds and then it would die.

I bought the Hobbyzone AC wall charger and used that to charge my battery instead. The AC charger gave me a more consistent full charge and I finally got my Aerobird off the ground.