Hmmmm....sometimes I have a problem with the ruler slipping as I get to the end of my cut - especially if I get in a hurry. So I slow down & sort of 'walk' my fingers down the ruler, pausing the cutter while I do. Another thing is to use the pads of your fingers, not your flat hand to hold the ruler in place (think of a spider walking on the ruler). Also, make sure your fabric is folded straight too. I hold it up, selvedges together at the top, & slide one side or the other back & forth until it hangs without a 'twist' in it. You may find that there is a big difference where the cut edges end up that way, due to the fabric sliding as it is rolled onto the bolt. (Another reason to buy a little extra if you can!)
I lay it down, then fold it one more time, bottom fold up to the selvedge, then line up the marks on my ruler to the bottom folded edge & true up the right side (since I'm right-handed), then turn it over so that the bulk of my fabric is on my right & make my cuts. I also recut to true it again after several cuts if it needs it (& it usually does for me LOL)
Hope this helps Peggy!