Tall grass Doesn't cut all the tall grass

The deck & blades are designed to cut a maximum of 4" off the top of the grass.
So anything taller than 8" is beyond the design capacity of the mower & deck.
cutting a trail through the savanah is what things like Brush Hogs are designed to do.

As far as less grass being missed cutting shorter, that's because the lower the deck is, the higher the suction lift is. My mowers start loosing suction over 3.75 inches or so, but it depends on the lawn. I can cut higher on dense turf with good results whereas on scraggly turf there would be missed blades sticking up all over.

I have a different mower, but concept should be the same. Tall grass wont cut right unless you raise the deck to a certain height. Change direction and re-cut the lawn again for a nice smooth look. Or just run it over and leave it looking like a pile. Other reasons could be clogging, low tire pressure on a side or uneven deck. Wet grass has an effect as well as if your bagging or mulching. Blade is new so that should be passed.