It could be the problem, have you grooves and it is no longer "healing"? if so time for a new mat.
I have looked at my Mom's mat , and it has REALLY big grooves in it as she uses the lines on the mat for the cutting line. It no longer heals and she has the same issue with blades. But I just can not seem to get her to buy or use a new mat she Says "there still some life in that mat".