Hi I have the same problem. so I took the plate off the
gear end and found that the screw had come off from
"a round bit with two holes in it" there is, what seems to be a counter rotation system with two wheels controlling the up and down of the two blades.
Unfortunately I dont know which way is the correct
way to re-assemble them, and I am awaiting an answer
from this page and also from Ryobi the manufacturers.
If I get an answer I will post it ...MIKEDEKITE
MIKEDEKITE, July 2008