If you have black-on-white then try adding -negate after the source image, it'll turn it to black-on-white.
Another approach I took recently was to combine this with -threshold (90 worked well in my case) and the Dilate morphology - I had a black and white set of plans printed using varying-sized dots.
Another approach I took recently was to combine this with -threshold (90 worked well in my case) and the Dilate morphology - I had a black and white set of plans printed using varying-sized dots.