Digital BW, The Print

I've been using MIS VM-Inks with a MIS CFS for a while now. Very good
results. But I've been noticing that the black ink bottle empties MUCH
faster then the other three.Actually,the other three are almost at the same
level since I started...
The lines are feeding normally when I run cleaning cycles or turn the
printer on,,I'm printing RGB files with the driver's "Color" setting. What\ufffds
up?

Andr\ufffd