The first thing I would do, assuming that the blades are the correct blades of the correct length, is to check the tilt of the deck from front to rear. With everything shut off and the deck in the normal mowing height, blade straight forward, mower on a flat surface, take a small block of wood (like a 2x4 short peice)keep it firmly on the ground and rub it against the cutting edge of the blade on both ends of the blade. Compare the two marks the blade makes on the wood. The front of the deck should be just slightly lower than the rear. Do this for both blades. If there is a large difference in height from front to rear, that can leave an uncut area in the center and on both edges. If that is not the problem, I would look for a bent deck or broken spindle.