You have to loosen the nuts behind the knife blocks before those allen screws will be able to raise or lower the height. They are only meant as a fine adjustment for dialing in the height. (So the tip of the knife at the highest arc of the cutterhead revolution is in line with the height of the outfeed table, as measured, for example, with a steel straightedge or rule.)
You'll need a small open end wrench to loosen the nuts on one knife only, then replace and adjust the heights, then retighten, then move to the next knife, etc. As you retighten, be careful not to tweak the knife out of alignement, and tighten back and forth across the cutterhead, rather than in a straight line from one side to the other, so the torque is spread evenly across the cutterhead...
Here's a link to a parts guide and owner's manual if that would help you out...If you don't find one there check here and maybe there's something close enough to guide you through the process....
Be careful...If those knives aren't properly tightened you'll destroy the beds, and make sure you unplug the power to the machine before working on it.

I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
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