Could be several things.
Most likely: Rolls are out of alignment. Does it curl the metal when
you just roll metal by itself? If so, it’s the rolls. Easy (ish) fix.
(To the extent it can be fixed. If the mill itself is junk, you may
never get it perfect.) I did a thing for my website way back, so the
instructions for leveling out your rollers are here:
Next possible culprit: the laws of physics.
Unless you’re using one of the steel pattern rolling plates, the odds
are pretty good that you’re going to have uneven thickness of 'stuff’
to roll. More ‘stuff’ equals more stretching of the metal under that
area. Which means it expands more, and causes the rest of the sheet
to curve away from that area. You can actually take a straight piece,
and turn it into an “S” just by having more stuff on the right at the
bottom of the sheet, and more on the left at the end. (Take a look at
my explanation of how the metal curves in the ‘realign your rolling
mill’ link above.) Same idea: the sheet curves away from the area
that got squished most. More thickness of paper to pattern with more
squishing, and more curving.
Roller printed metal never comes out perfectly straight if you do
anything serious with it. Which is why you always want to pattern a
piece larger than you plan on using: to make sure you have enough to
cut your finished piece out of, regardless of how it bends.
PS"> you shouldn’t be rolling wire on the flat rolls of a mill,
especially a quality product of the Middle Kingdom. You’re likely to
crush a print of the wire into the rolls. They’re case-hardened, and
the increased PSI (by way of reduced surface contact on round wire)
can get over the pressures needed to deform the cased layer.
You can hammer it mostly flat, and then use the rolls to even it
out. That’s OK. But I always do that over against the very edge of
the rolls, just in case it does print. That way the rest of the roll
is still useable.