Check the tire pressure and position of the anti-scalp wheels, they should be approximately 1" off the ground when the deck is in the cutting position. Rough terrain, too low of a height of cut, and too fast of a ground speed can also contribute to lawn scalping. Check the owner's manual for the correct air pressure in front and rear tires.
If these suggestions don't correct the scalping issue, please contact an authorized servicing dealer in your area for evaluation and repair.