Turn off the mower completely and clean out beneath the deck with a mower-cleaning tool to remove any built-up grass deposits. As debris collects inside the deck, the blades can't move freely and you'll have unusual patterns or unevenness forming in your lawn as a result. The mower-cleaning tool scrapes this debris off and restores the deck to proper function.
Measure the height of the mower deck with a ruler when the mower is on flat ground. The deck should be an equal distance from the ground on all sides to ensure an even grass cut. Adjust the leveling bolts on both sides of the deck as needed to readjust the height until it's even. If the mower hits something like a rock or a large stick while mowing, it may knock the deck off balance.
Inspect the deck for any dents or warping. Use a rubber mallet to carefully straighten any dents in the deck if the deck is metal. Plastic decks generally need replacement if they crack or warp.
Remove the deck for a more thorough inspection or cleaning as needed. Remove the grass catcher and take off the pins or mounting bolts holding the deck in place. Lower it to the ground and pull it out from under the mower. Unhook the cutting blade from the underside of the deck with a wrench and set it aside.
Sand off any rust with sandpaper and use the mallet or a hammer to eliminate any remaining dents or warps.
Reinstall the deck once any corrosion or dents are fixed.