-blades worn? (rounded tips?) Blade bent?
-Are you mowing at wide open throttle? If so, move your throttle lever on the dash to full throttle, then check the lever on the engine and make sure it's up against the stop. If not, adjust it until it is.
-Belt slipping? Check condition of belt and pulleys. While you're at it, check the gearbox fluid and check it for leaks!
-Spindles nice and tight? To check them, remove belt and try to wiggle the blades. If one or more spindles is loose, replace the bearings and try again. If you have to do the bearings, make SURE the new bearing fits tight in the spindle housing-seen many really loose.
-Is the grass dry? You can't mow wet grass effectively.
Is the bottom of the deck clean?
-Are the blades correct for the mower? You wouldn't believe the difference an aftermarket blade makes. I've never found a non-Kubota blade that will cut worth a crap. Not even Gator blades and a lot of people swear by them.
-Does it have a mulching kit installed? If so, mulching requires special care to get any kind of decent quality cut!
-Is the deck level, both side to side and front to rear? Before checking it, adjust the tire pressures....
On the deck level, I suggest checking the front-to-rear by measuring the front of the center blade and the rear of one of the side blades. Much more accurate than the "lazy" way of measuring the front and rear of one blade (usually the right side since it's easy to get to). And while on that subject, it's not accurate to measure the deck shell, the shell doesn't cut the grass the blades do. And for that reason I have MUCH better success in measuring at the blade. I use a J.D. blade gauge. The Kubota one is a lot nicer but also a lot more expensive.
Don't know where you're at but if you have those little "sprig" things, like grass that grows a foot overnight and is usually like a single stalk, then I don't know what will cut that stuff....I have a whole yard full of it and even with a great cutting mower I still have to go over it twice to cut it. Some call it Johnson Grass, but I don't know if that's the proper name for it or not. I call it tough grass.
That mower cuts extremely well, but only if everything's up to par. I would say it's got almost as good as cut quality as the John Deere 44" Piranha decks had....they were the absolute best cut quality, IMO, but also a pain in the rear end with their 6 blade setup and plastic under-deck baffling.