You may well have an oldish box of paper, as Innova have recently addressed this problem (which is fairly common to all FB papers) with a coating on the back of the paper.
Trying to flatten it under weights doesnâ€™t really work, so possible remedies are to carefully bend back the sides, and especially the corners of each sheet by hand before feeding it in. You canâ€™t really bend it too far, as convex to the printing side is ok, but concave leads to serious problems, like head strikes.
The other remedy is to make yourself a de-curler (or you can buy one for Â£200!) by taping a large sheet of stiff paper to a length of 1Â½ plastic waste pipe â€“ unroll on a flat surface, place paper face down, and roll it up; hold for 30 secs, then unroll.
Even better is Ikeaâ€™s Â£6 plain white stiffened cotton roller blind, because unlike paper it doesn't have a 'memory' and wont keep rolling up on you once you've used it a few times ... and don't worry about any very slight ridges across the paper; they'll disappear once it's been through the printer.
This device is also useful for de-curling roll paper after printing.