Remove the fingers, check the retaining spring. Clean the spindle bore and lightly grease with graphite grease the pivot point as per OEM instructions.
I think the fingers are sticking, the power can be much reduced if dirty. Also, check the stroke, there is a distance from the spindle face to the hsk body in clamp and unclamp states. Release centre grub screw and rotate to adjust the length, then pinch the grub screw again.
Until this is done, power measurement is pointless.
I've had dirty hsk drawbars reduce from 27Kn to 18Kn just by being dirty. (Also making clicking noises because they stick before releasing). By the way, if wrongly adjusted or sticking, the tool may get jammed in the spindle, so be careful.