How is it set up? Dual cmyk, 6 color? What order are the cartridges in?
Mine is set up in such a way that there is no way cross contamination can occur. 4 heads, each head handles it's own color.
If I'm not mistaken this printer can be set up 2 different ways using 4 color.
B,Y,C,M,M,C,Y,B
or like mine (which is not listed in the manual)
B,B,C,C,M,M,Y,Y
or 6 color
B,C,TL,TL,Y,M,Lc,Lm - this is what I am assuming yours is and if so the previous poster probably hit the nail on the head with cross contamination. This indicates to me that head '1' on the left is handling black and cyan. Could also be a bad pump that handles that head.
The ink lines are in the same order meaning the first two colors share the first head on a 4 head machine and so on.
Assuming you have a 6 color setup the 1st head on the left would handle black and cyan, (2) Transition Liquid, (3) yellow and magenta, (4) Lc and Lm.