When you are washing it do not scrub back and forth.
More like place bristles on fabric and rotate brush head slightly without moving bristles too much. Lift and repeat in next area. By doing it this way you're..., I dunno..., "drilling" the bristles into the indentations caused by the weave of the fibers to loosen dirt. I think the dirt is accumulated mostly in those indentations
Scrubbing back and forth will only "rake" the bristles across the upper-most fibers, wearing them out and yet leaving dirt in indentations.