Chances are on of the screws that attaches the bottom blade to the underside of the chassis has worked loose. Tip the mower back onto it's handle and check both screws. If loose, then loosen off the cylinder adjuster screws (the ones which you have already tightened right down) and tighten the bottom blade screw(s)- then raise the mower upright again and then re-adjust the cylinder adjuster screws. Brown paper should be used to test the cutting efficiency of the mower as this best replicates grass.
pleasant
August 2008