With lots of floatation down low, and all of the weight up high as weights on a tankband, it's to be expected that the most stable position will be supine (back down, face up). That's the position where the lead is below the floaty body and wetsuit. With all the weight on your back and on the tank cambands, a normal horizontal (in roll direction, not pitch) orientation will be an unstable equilibrium. OK if you stay perfectly horizontal, but as soon as you roll a small amount either right or left, then you'll tend to keep rolling until face up.
Does this describe your problem?
If so, then you need to take weight off of the cambands and move it to a weight belt or some other method of getting it more towards your belly side.
If this doesn't solve it, you may need to go to an aluminum plate so that you will have more lead that you can move to your belly, but that's not likely. When you go to saltwater, you'll need about 6 pounds more weight, If you put that on your belly side, the problem may go away.
Once you fix the roll problem, then you'll be able to work on any head up/head down pitch problem.
BTW, the worst case condition for turtling like this will be when you have a near empty tank, are shallow, and you have no air in the wing. If you are at depth, or if you are overweighted to the point that you have to add air to the wing, then this will add stability.
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