Is the blade sharp? Is it on upside down?
Yeah it happens. we pick up the blade and reach under the mower to mount it and bingo. Kinda like the backwards chain trick!
I've found different blade types/shapes may cut better with the different grasses.
Strait blade with mild lift wings, or wavy mulcher blade or the gator types all cut a bit different.
My Snapper Hi-Vac deck will bend tall grass over and skip it, due to how far inset the blade is into the deck.
Thus I had to mow at half deck with cuts in tall grass.
As an easy remedy, putting a 19 inch blade under the 21 inch deck actually a wider swath than the 21" blade does.
Reason being that the 19" doesn't ride over the bent down grass but cuts it due to the larger gap at the blade ends letting the grass rise up a bit and get cut.
Seemingly minor Blade changes can make a major difference in the quality of cut.