On a more "elementary level" I have found that in some cases "biting" with the embouchure over time causes the reed to bend toward the tip of the mouthpiece. This makes the mouthpiece play as if it has a smaller tip opening. A quick way to diagnose this is to insert a business card between the reed and mouthpiece as far as it will go and leave it there for about a minute. If the reed plays much better afterward, that was the problem. The solution, of course, is to open the teeth more and push in with the corners when you play.
Another common cause of a good reed becoming unresponsive is that the fibers on the bottom of the reed swell and expand where they come in contact with moisture. The solution I was taught is to remove the reed and use a Reed Geek or another perfectly flat blade to make several strokes from the butt of the reed to the tip to make the surface flat again.