Firstly it looks as though your rollers are not EXACTLY parallel.
Always roll in the centre of the rollers. Though perhaps if the
curve is slight you can overcome this by rolling more gently and
reversing sides after each pass. Then for the last pass release the
pressure a bit and roll, check the thickness and reverse sides and
re-roll if necessary. Too much pressure can cause problems. Re
alining your rollers can be tricky, but not too difficult.
David Cruickshank (Australia)