To me, this sounds like an injury to her feather follicles on the "bad" wing.  Doesn't make sense that one wing grows beautifully and the other doesn't, except for that, especially since feather molt usually happens on each side of the body at pretty much the same time.

Maybe a vet could test a feather and see why this happens.

Do you yourself clip her, or could someone have clipped her too closely and injured her?

Annie

I have both had her clipped by someone else & have clipped her myself.  When I clipped her myself, though, it was always to trim off the 'stray' growing feather so that it matched the previous clip line (which was done by a bird groomer).  Never thought of the clipping/injury aspect of it.  Her broken off wing sprouts usually are VERY tapered at the ends and almost look pinched, like the shaft has stop letting the new feather grow (looks like a normal sprouting feather towards the "free" end of the feather).  Does this make sense? Yes, I agree, strange that it would be happening on one side and not the other.. injury sounds like a likely cause.  But, now I just wonder what to do to help her fill out the "bad side"...she flaps & flaps & feels the lift on the good side and, thus, I guess makes her feel like she can take the plunge.  She seems SO ready to fly and it breaks my heart!

Do you ever gently pinch the tips to release the keratin coating at the ends of the feathers? I used to do that occasionally on wing and body feathers if one looked like it was stuck. Especially on the tops of their heads, mine will sometimes seem stuck.  Probably would finally release but they seem to like the little pinches.

A vet could pull one of the encased pinfeathers and see if the shaft looks normal where it is inside the follicle. 

Annie

I always do the wax on her neck & head feathers.  She doesn't allow grooming on her parts that she can groom herself.

Sounds to me like she might also be biting her feathers off.   Dianne