technique doesn't sound like the problem, but is it innacruate, or hold sensitive ? If you shoot it as you described, with a very consistent hold, and it's still innaccurate, it's not hold sensitiive - it's innacurate !

So now to answer the why is it innacurate..
i) give the barrel a good clean
ii) try a few different pellets - JSBs, H&Ns, superdomes are all worth a crack - alowwing a couple of dozen shots each tiem you switch got the bore to adjust to the lead composition of the new pellet
iii) shoot over the chrono - is it consistent - if not may need a new piston seal or breech seal, or a busted spring
iv) knackered scope - try another
v) let someone else try the gun
vi) the barrel may have a problem - e.g. crowning.
vii) come to the springer bash (actually, just do that anyways) !