Digital BW, The Print

Just when I thought this was coming right, I now have zero ink coming through the K channel.  Also, I can see a couple of bubbles in the centermost ink line which I assume is the K. Any ideas?  

I thought printing a pile of pure black pages might help but it doesn't seem to have helped.  

Is there a way to slide the printhead across and sit it on a paper towel soaked in windex?