Resistance to blowing

by Bill Scherler
(Somerset, NJ )

I have been playing clarinet for about 7 years and progressed from a Bundy band instrument, to E-11 Buffet and now R-13.
Generally I am getting good tone and response from the instrument however suddenly I get no sound at all. It seems like the air passage is blocked. It as if the lower register and upper registers are fighting each other. I have to stop altogether and get the air started once again. This is very disconcerting to say the least.
Could it be my embouchure is breaking down ?
I check to make sure all tone holes are covered
Not sure what is causing this to happen.
Can you offer any advise
Thanks, Bill