Without more information it is hard to answer your question. If reeds suddenly stop working, it may be because you are "biting" too hard with your embouchure and the reed begins to bend toward the mouthpiece and close off after playing a while. The way to tell is to take a business card and insert it between the reed and the mouthpiece until it stops. Then play on the reed again. If it is much improved, you are biting and closing off the reed.
If this is the case, the solution is to feel as if your jaw and chin are pulling down and away from the reed when you play. The pitch on the mouthpiece and neck alone should be and Ab concert on alto and E concert on tenor. If the reeds you are playing are named after a water fowl that may be the problem as well.