OK,
If the flail mower does not sound like a shop vac vacuum cleaner it is not creating the amount of
pressure gradient/suction to lift the clippings up and over the flail mower rotor and the flail mower
rotor is not spinning at 2,200 RPM+.
Tall, thick, dense brush requires the user to operate at the lowest forward travel speed
at the lowest mower setting with the engine at the 540 RPM rear PTO engine setting.
Tall, thick, dense brush mowing using a small tractor requires the user to operate the
flail mower in an overlapping mowing method to recut the material and catch the
stringers/stragglers.
What is the low range speed on your mule?
Is your flail mower equipped with side slicer or the scoop/spoon knives??
A flail mower operates best with the roller at its highest mounting point (lowest mowing height setting)
which is the top mounting/height adjustment holes in the side weldments of the mower.
The skids should not touch the ground, their only job is to protect the flail mower rotor, knife hangers and knives/hammers.
Are the V belts on your flail mower tightened by hand using a flat snubber pulley?
If you are mowing 20 acres only occasionally you will not control the invasive plants like ragweed, water hemp,
thistle, poison ivy, poison sumac very well and mowing will be harder to do.
If you mow the 20 acres in 4-5 acre blocks every 2 weeks at a low cutting height you can manage the mowing much more easily.
You will be better off using the Massey Ferguson 135 for this mowing if it is in operating condition.