If you sew a lot of synthetic fabric, the blades become dull after about 20-30 hours of cutting. Apply a little more pressure usually prolongs the life.
Don't cut paper or anything else with the same blade you use for fabric! Or you will dull the blade quickly for fabric. (Hint: once my blades don't cut fabric well, I use mine for paper for months!)
Package of 5 replacement blades. Blades are oiled to prevent any rusting.