Hopefully you are not in the same situation as I was in 2007. My Epson 4000 had to be checked by a repair shop because of a bad deflected nozzle case in the matte black channel. It was concluded that the printhead needed to be changed, which sets you back about $700-900, depending on the repair shop. The deflected nozzles are easily seen as gaps in the ink squares that are printed when one does an automatic ink check on the machine.
More discussion on this problem can be found
here, but here is a summary:
What has worked well for all but the matte black channels is the advice that I received from a professional printer after going to the repair shops (and obviously choosing not to go through the expensive repair, which are just not worth it). His insight was that matte black was the most likely to create troubles because of the larger carbon particles in the ink, followed by the magenta and yellow inks.
To unclog the nozzles, his trick is to park the head on top of an absorbing pad of lint-free material (Trader Joe's sells a lint-free cleaning viscose pad that works great for that). I cut a 2x3 inch piece and placed it over a larger double layer of Saran foil to stop any ink from leaking to the rollers. Then, slowly move the head from left to right to cover the pad until you see a piece of the pad on the left edge of the head. By parking the head overnight, the the ink will slowly seep through the nozzles, keeping them wet and dissolving most of the troublesome deposits. You may need to do this for 5-7 days, changing the pad every day. The head can be parked off the middle of the carriage without problem. If you don't know how to unlock it, there is a lever near the front top next to the cutting blade that needs to be slightly depressed. You need to go under the plastic cover to the right of the machine to find it. Don't worry, nothing will bite you! ;o)
Hope this will help!
Yves