Could be a couple reasons why this is happening.
First thing I would check: are the fly lines coming off the kite on the correctly....as in...is the right fly line in your right hand, and is the left fly line in your left hand. If these are reversed, the kite will not fly.
Second, is the bridle correct? Check to see that it is 'even' on both sides. Meaning, as the line comes off the frame, is it the same distance between each knot/connection on each side. And when you hold the bridle by the knots (the knots on the pigtails where you connect the fly line to the bridle with a larkshead loop) and hold the kite up so it's laying on its back, the kite should lay flat/hoizontal to the ground.
Third, is your fly line even length on both sides?
check these things, and give it a try again
Good luck!
Theresa