It is a riding technique issue, not a bike issue. If you are trying to get traction with the rear, the BEST traction occurs when the front end IS light. If you shift too much weight to the front wheel then you'll unload the rear.
Sitting while climbing is the biggest cause of your problem though. Try standing, and let the bike come to you with soft arms. This will let your weight shift naturally to the rear tire and keep you balanced front to back. If you lock your arms, then your body mass will begin moving backwards when you start climbing the hill and will bring the bike with it.
By sitting you really lose the ability to shift your weight correctly both for going up the incline and steering on the incline.
Try it standing, let the bike come to you, and stay balanced vertically like a tree would be growing straight up towards the sky on the incline. Change lines with your feet steering the bike, not the bars. Keep a light touch on the bars and planted on the pegs.
Do your accelerating before you get to the hill while you have traction, then hold steady throttle on the hill. Shift your weight front-to-back gently to keep the front end light and the back end biting, but not by pulling or pushing on the bars. Use your torso muscles against your legs.
Your best traction and climbing occurs in most cases (not so on sand) when the tire is not spinning. Try to work at feeling the connection between the rear tire and the ground, and use as little throttle as necessary to maintain the traction a few times. Less throttle with no spinning will take you farther than more throttle with alot of wheelspin (except sand).
Lemme know how it comes out.
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TexKDX