Digital BW, The Print

I have a 4000 loaded with carbon sepia piezotones and 3 of the K7 
inks.  I started today with a perfect nozzle check.  After 4 perfect 
prints, the 5th one was terrible.  I did another nozzle check that 
showed a completely blocked piezo nozzle.  After 3 head cleanings it 
is still not putting out ink so I turned it off and am letting it 
sit.  I print at least once a week, usually more, and I shake the 
carts weekly.  This happened once before and I got it to work the 
following day.  Any ideas what caused this and how to prevent it in 
the future?

Thanks,
Jenny