You have to adjust fifth line length. I bet it is too short. Park the kite at 12 and pull the bar in all the way. The fifth should have just a tat more slack then front lines. On the other hand if you make the fifth too long you will lose power the kite will feel gutless.
Many years of flying Rebel, I don't agree (maybe true for other North models, Dice Vegas etc.)
On Rebel the 5th needs to be tensioned, not have more slack than the other front lines.
5th too long, you get less upwind and jumping performance
5th too short, kite feels gutless and on/off (depower is not smooth), heavy bar pressure
An keep in mind that it's Y BRIDLE AND FRONT PIGTAIL SHRINKAGE as well as bar lines affecting this. So you need to measure the bridle as often as you measure the bar lines (Armin made a great post a while back explaining why Dyneema shrinks)