To fix this problem, try stretching the fabric. That just might loosen up the stitches and allow the piece to lie flat. If the corners begin to curl again, make sure you're using the appropriately sized hook for the yarn you're working with. Switching to a larger hook size will create bigger stitches and loosen up the fabric, allowing it to lie flat. If all else fails, you may need to adjust your tension by changing the way you hold the yarn.
Sometimes you won't realize that the corners of a project are curling until you're all done with it. Instead of tossing your work away in a heap of frustration, try blocking it. With some yarns, such as wool, blocking loosens up the fibers, allowing you to stretch out any curls.
One way to prevent curled corners is by simply loosening your stitches. If you find that your hands are cramping up, chances are you're holding the hook too tightly and therefore creating stitches that are too tight as well. Relax your grip for looser stitches.Making a gauge swatch helps you identify and prevent any potential problems early on. If your swatch is curling, then your project will probably curl as well. Always make sure you're working to the gauge specified in your pattern if you want to wind up with a successful project.