Digital BW, The Print

I'm having a problem getting one spot (one 2mm piece of one line of the nozzle check) on one channel (carrying MIS Eboni black) to print. Problem has persisted for a few weeks. It is not apparent on prints--no corduroying. I have tried:

• daily repeats of four back-to back nozzle cleaning cycles separated only by a nozzle check.

• cleaning under the heads by passing head over paper towel spritzed with MIS flush fluid (repeated)

• letting the head sit awhile over paper towel soaked with flush fluid, then cleaning. (repeated)

Would appreciate any ideas on what to try next. --John