If all is as you say... I'd be guessing that something slid back into the tail
and made it excessively tail heavy, or that you have developed a warped wing.
After the first flights... did you reshrink any of the wing's covering?
Did the wing sit in a hot place, that wasn't flat? (tips resting on something)

There's not a whole lot of things that can cause what you describe.
If you honestly can't think of anything that has changed... recheck the CG,
and carefully inspect the wing itself for any warpage or wash-IN.
Make sure the leading edges are at the same incidence, as well as the trailing edges.
Make particularly careful inspection at the tips, when you're checking it out.

If it's not something like this... and your tail surfaces are tight, and the wing is tight,
then I'm at a total loss, unless you're just not getting enough airspeed before liftoff.
Either that... or you've got a stray mix in the radio, that you're not catching in your preflight.


Please report back, whenever you nail this one down