Hey dude, if you shorten the back lines too much the kite will stall.
Lots of people make the mistake of trying to power their kite up and they don't sit in the front of the window enough. Make sure all the lines are the same length and sit the kite at 12 and stay stationary. Pull the bar in and if the kite flies towards the trailing edge at all then depower the kite or lengthen the back lines.
The most effective way to fly kites is fast. They generate more lift (apparent wind) and fly a lot better. So build up some speed first and then edge harder.
I have seen guys cursing that a 13m is not power enough will I am on a 9m in boots. Technique is sometimes half the issue. The rest is set up.
Also you need enough wind for any kite to fly well
Hope this helps