Have a look at these: if you've built with Kwikstage, you know what's wrong. I've tried loads of fettling and naggling. And you can see that I've thought of a 'fix', but 'ees a bodge, and I no like 'em.
Not a pretty sight is it?
Honestly, I must have spent an hour fiddling about: I took it all back down and started from scratch. I measured the diagonals - all within a few mill.
Whas' goin' on eh?