Had the same issues on grass as you, would not rotate and lift off smooth , I added spacer in the nose leg to stiffen it up, eventually kept putting more take off flap in and this helped it lift off better, I was going to try to fit weaker springs in the main legs to it sits down at the back more I'm sure this would help it no end.

unfortunately after 5 flights I add a high speed stall it recovered but clipped a tree and is totalled. I've ordered a replacement and intend to lengthen the nose leg this time and cut the fuze to allow wheel to retract.
hope you get the better of it as they are such a nice plane to fly, other than the high speed stall mine flew fine it was just the take off I think JL should have designed the angle of the plane to sit flat.

Brian