When you have to start really putting alot of muscle into cutting the fabric, I'd say its time for a new blade.
I use a friskars 8mm, I tried to use the bigger cutter I have a small hand and it just didn't work for me.
One thing that is maddening is none of our walmarts carry 8mm blades anymore. I get two blades for $4.00 from hittle sewing supply. I just get $20.00 worth and that keeps me going for awhile.
Keep it rocking,
Donna Mae