Digital BW, The Print

I received the new spongeless 1280 carts a few days ago; filled them
up according to Paul's notes, plugged them into my printer.  Nozzle
check was fine, prints (PK ink on Ilford Smooth) looked great.

Now I am having trouble with dropping nozzles.  I tried refilling and
resetting the "color" cart - no real help.  I am particularly having
trouble with the "magenta" position - the center one.  If I can manage
to get the others working (pull the cart and purge the port with the
shortened adapter), the magenta will get maybe one layer of lines on
it's nozzle check, then nothing.

Is there something I could be missing here?  Is there some way I could
have plugged the air vent (maybe overfilled or sloshed some ink into
the channel which dried overnight) - could this cause the problem?  It
seems strange that
the one channel is giving so much trouble.

(Plugging the Epson cart back in results in instant, perfect nozzle
checks, FWIW.)

Thanks for any help!
Eric