Periodically check your rotary cutting rulers for damages and consider repairing replacing if needed. Check for cracks in the acrylic, and if possible, take the ruler to a glass cutting shop to have it cut down with a laser cutter to remove the cracked area. Also check rulers to make sure the printing is still in good condition and hasn’t worn away at the edges. If the screen printing is faint or worn completely away, it may be impossible to cut fabric accurately. When you can’t cut accurately, new rulers are in order!