There are a few things that can cause marcelling or "washboarding". If you look at the specifications in the owner's manual it may show the clip rate or frequency of clip FOC or something like that. The ideal height for a reel mower is close the the clip rate. You wont be able to mow much lower than about 20% lower than the clip rate without marcelling.
Another thing to check is all the blades should be cutting or pinching a thick piece of paper. If one blade is bent out and contacting the bedknife while the others have a gap between them and the bedknife that will cause marcelling.
I'm guessing clip rate. I think McClane sells a push reel mower with a small clip rate capable of cutting very low.
Regards,
Roland