QTR-Quadtone RIP

I've just installed NK7 inks in my Epson 2200 and have a huge problem
with posterization in mid-tone areas, though my shadows and highlights
show good detail. There is no distiction in tones from about a level
of 162 to 192. I did a nozzle check with my Epson inks before making
the switch and the patterns were all fine. When I repeated it with the
NK7 inks the area that would normally be yellow is very pale, but
visible. The light magenta slot is slightly less pale, but the magenta
location shows no visible pattern (at least to my eye).  I tried
cleaning the heads and the pattern remains unchanged. Any help is
greatly appreciated!

Bob Spitz