welcome aboard jeannie. i use both olfa and fiskar cutters in all sizes. so long as the blade is sharp i can always cut through at least 4 layers. sometimes with a sharp 60mm i can even get 6 layers.
my friend has trouble because she doesn't keep the blade straight up and down next to the ruler. be sure it is not leaning when you cut. get up over the top of the piece so you can really see where the blade is and bear down evenly as you cut.
hope this is helpful. keep quilting