I'm guessing inconsistent seam allowances (a problem I struggle with!). I've found two things helpful for me. I found a tool that has seam allowances with a hole at each measurement. It's red and looks like a ruler, about 6" long. I put my needle through the quarter inch hole, put the presser foot down, and then make sure the ruler is straight. Then I align a strip of thick mole skin along the edge of the ruler and try to position the needle so the edge of the presser foot is also at the quarter inch mark. It helps me keep my seams fairly consistent and helps me sew a little straighter than I tend to do free hand.