I don't know anything about Summa, but with cutters in general, the first thing you look at is, I know it sounds silly, but is the bladeholder installed properly? Secondly, have you checked the extension of the blade? If you peel your media away from the backing and compare your extended blade to it, it should only be extended the width of the media (not the media + backing). If you need to crank up the force insanely high, it's usually because you're using the wrong blade, wrong bladeholder, or wrong blade extension for the media you're cutting. Beyond that, most of these cutters I've seen have teflon cutting strips. If they're worn out, the media can sink into the trough in the cutting strip (causing inaccurate cuts) and causing you to increase the force (which further degrades the strip and puts more wear on your motor). Check that out too.
Also, make sure the down force isn't being overwritten by the software you're using. Sometimes folks will accidentally check a box on their software and not realize what they did. You can set the force on your plotter all day, but if the software is overwriting it, you won't get anyway.
Beyond that it could be a solenoid issue, or a cable issue going from the penblock to the main board. You may need a tech to come onsite and start swapping parts out.
edit: also, make sure you have the latest firmware installed. If all else fails, do a factory reset.