There are different flail mowers.
One has a larger number of more delicate
blades for a finer cut, and one that has a
lesser number of more rugged heavier blades
to cut heavier brushy weeds and light brush.


I'll say here and I don't know the particulars
of the poster, that many people tend to try
and mow too fast. You don't get as good of
a cut and your mower/ tractor will bounce
around and not cut evenly or as well as it
would if one would drop down a gear and
back off the throttle a bit.
I have a neighbor that rushes through and
bounces his tractor and shredder all over
the place and has clumps sticking up here
and there all over where his machine was
slightly airborne