I am having a problem with my Fiskars Rotary Cutter (the one that comes with the cutting mat and 6x24 ruler). The nut doesn't stay tightened. It loosens up after only about 5 or 6 cuts. Sometimes I'm afraid it will come right off. I love the angle you hold it at, not like the others I've seen. I was thinking it might have been an issue with my specific cutter, so I went and bought another one just like it. Once I change the blade, I have the same problem. I have been very careful when taking it apart to make sure I'm putting it back together correctly. Has anyone else had a problem?