Did you wash the fabric first, or at least open it right out and make sure the selvedges were correctly aligned?
You get the V-shape cut if the fabric isn't folded on the straight grain - try it out on a bit of paper and you'll see (but don't use the cutter!). When the fabric is folded and wound onto the bolt it is not always straight, and any starch etc in the cloth will tend to distort it. This is why pre-washing of patchwork and quilting cottons is recommended as any dressing in the fabric comes out and you can fold it along the grain, matching the selvedges. You will probably find with a crosswise fold that the cut edges will no longer line up. Trim a straight edge once you are satisfied that the grain is as straight as you can get it, and cut carefully. Walking your hand up the ruler will help keep it from slipping - cut, stop but don't lift the cutter, move your hand, then cut some more. it seems fiddly at first but you soon get the hang of it.