Don't know if anyone else has run into this but thought posting may benefit someone in the future.
About a month ago I switched from matte to gloss black (after about 2 months of printing only matte), I noticed that the black was not printing on the image at all. I went through 4 cleaning cycles and finally it cleared up so all was good- but it used a lot of ink.
Yesterday, I switched back to matte black and same thing happened. After 3 cleaning cycles, I was only seeing sporadic black squares on the printhead test print and it wasn't improving. It occurred to me that maybe something was getting stuck in the ink tank vs. in the lines. I took out the matte black cartridge and gave it a shake. The test print came out perfectly this time.
So, I will probably just give the black cartridges a quick shake before switching now going forward. Hopefully will save me a lot of ink! Also, if you use ink faster than me this may not be a problem for you. Both of my black inks have been in the printer for almost a year now.