It is not so much complicated as that there are a lot of variables, and you need to isolate one at a time.
Corrected eyesight (more of a problem than people with good eyesight realize - a loupe does not correct for astigmatism)
Screen - right position, right way up
Mirror - displaced (unusual, but possible)
Lens pair misaligned/not shimmed
Lens board not parallel to film plane
A second lens helps isolate the lens pair as the problem.