To my mind the simple truth is porbably that after a while playing the lip is becoming tired and maybe the diaphragm isnt supporting as it should...at this point a little tension is creeping into the embouchure thus closing the reed to retain pitch..It happens to me and has nothing to do with any particular reed makers reeds, I have found it can happen with anyones reeds, sorry to say it but the smallest pressure which comes from the embouchure will do this quite quickly ..
My reeds close now and then and I just give them a gentle squeeze and they are fine..Always give a reed a little time in water and relax the embouchure..I am afraid to say that we ( none of us amateurs and students I suspect) support correctly all of the time..If a reed was playing in pitch when we started then the fault is most likely to be ours and not that of the reed maker..so relax and keep the diaphragm support going..
Kindest regards
Andrew
