I'm knitting hats for charity, & one of the patterns I'm using is a straight stockinette stitch knitted on circular needles. The first two hats did not roll, but the last two are rolling at the base. Can anyone tell me what I may have done different? What to do to prevent this? How can I fix the ones that have rolled, other than ripping them out or crocheting an edge? Thank you, it's nice to have a way to get these problems solved/fixed!