One detail I didn't mention because I didn't think it was relevant, and because I knew what the reaction would be, was that these are 're-manufactured' cartridges.
I finally heard back from the vendor who said 'Have you removed the black tape?'
I replied that I hadn't noticed any black tape and that I hadn't taken it off any cartridges, but cyan was the only one that didn't print (except when in the black slot!)
So I found and removed the black tape and the problem is now solved.
I still don't understand why it would print in the black slot but not in the cyan slot, though.
I finally heard back from the vendor who said 'Have you removed the black tape?'
I replied that I hadn't noticed any black tape and that I hadn't taken it off any cartridges, but cyan was the only one that didn't print (except when in the black slot!)
So I found and removed the black tape and the problem is now solved.
I still don't understand why it would print in the black slot but not in the cyan slot, though.