Sounds like wing droop to me FJ and most probably due to the slippery velour. Also the bamboo fleece is very stiff in comparison so it most likely can't hold itself up IYKWIM.
You could make the wings a bit smaller if you think it may help. Something that's just popped into my mind is every nappy I have that is made from bamboo velour is side snapping. Have you made a side snapping nappy yet?