What they said- thread tension. When using a sewing machine, thread tension is primary importance. Next is using the proper needle for the thread, fabric, and type of sewing you are doing. Too much thread tension and your stitches will pucker the fabric and it will never lie flat. Not enough thread tension and the stitches won't hold the fabric firmly. Practice on scraps till you get your machine set up to perform like you want it. Keep the bobbin compartment free of lint. But I believe Im just repeating what others have said.