The more times you fold your fabric the more likely you are to get a bow in it. I used to have the issue until asking my grandmother about it. She had me show her what I was doing. The first thing she said was I was folding the fabric too much to make it fit my ruler/cutting board. I would end up with a bow at every fold. I went and bought a huge cutting mat and a long ruler. I have never had a bow again. Also like everyone else mentioned make sure you are squaring and straightening your fabric.