Curling inserts are caused by the different insert layers shrinking at different rates. Most brands now prewash the insert fabric prior to manufacture to avoid this, so hopefully if you buy a well established brand you will never face this problem. But if you bought second hand cloth nappies or older versions of a nappy then you need to watch our video which will show you ways to reduce the curl.
PS. if all else fails, you can iron your inserts to flatten them. We recommend doing this between two tea towels so that the iron doesn't touch your inserts directly.