MaryK .... Suggestion ... Take your not-so-good blade on your rotary cutter to Triangle and ask to see their manual blade sharpener. Of course, by taking yours, you can ask for a demo and see if it makes a difference for you. Nancy says she just runs it thru it a few times every so often. I said I was going to take mine in and let her prove to me the difference that it makes, but haven't done that yet. Let me know what you think when you go!
I think that with your specialized type of quilting, you do a lot of heavy duty cutting at times. So it could be that you are harder on the blades than many are. As suggested by others, it could be your mat is wearing out, and that can dull your blades faster, though it shouldn't put nicks into the blades ... but one never really knows, do they?
Nicks usually come from running into your plastic ruler or over a pin or some other obstacle.