If the mower isn't cutting the grass evenly, first make sure that the wheels are set at the same height. If a wheel is damaged, replace it to ensure that the mower travels evenly over the lawn.
Also check that the blade is sharp and undamaged. A dull blade tatters rather than slices the ends of the grass, leaving the tips brown and dry. A damaged blade is off balance, so it cuts unevenly. Replace the blade if it's damaged.
These repairs may help solve your gas walk-behind mower problem
Replace a lawn mower blade
A dull or damaged blade won't cut grass evenly. For safety and for a cleaner cut, replace a mower blade that's chipped, broken or otherwise damaged. If the mower vibrates too much, check the mower blade for damage because a bent or unbalanced blade can cause mower vibrations.
The lawn mower won't move with a broken wheel. A damaged wheel can cause uneven grass cuts. If the wheel on a walk-behind mower is damaged, replace it with one that matches the size on the opposite side of the mower.