Digital BW, The Print

I recently switched to the UT-FS inset from the MIS-UT inkset, on an
Epson 1160. First prints showed some posterization on EEM paper. So I
printed the same picture for three of the five curves and also a 21
stepwedges for each curves.

Grayscale-EEM Curve: Print did not show posterisation. Stepwedge print
clearly shows all steps.

RGB-EEM-1: Print shows posterization, stepwedge print shows blockage
at 35 to 45 and again at 60 to 75

RGB-EEM-2:  Print shows posterization, stepwedge shows blockage at 70
to 80

RGB-Mild-EEM: Stepwegde shows blockage at 70 to 80

Photorag-1: Stepwedge show blockage at 40 to 50 and again at 70 to 80

I have no idea on how to modify these curves. Any help would be most
welcomed. 

Also, what am I missing by using only the Grayscale-EEM curves ?

TIA,
André