Rider not cutting evenly - Garden

It's usually a bent blade, or a bent or loose deck, or a guide wheel or roller missing. It can occasionally be even a dull blade that pushes the grass over rather than cutting it. If the other blade(s) is sharp, it will cut at the correct length and you'll see an unevenness between the two.

Park it on a flat, smooth surface. A paved driveway or sidewalk works well. Check your wheels and/or rollers on the deck. Are they all touching the ground? Are they loose and wobbling? If nothing is obvious yet, take a tape measure and check the height of the bottom edge of the deck in several spots. They should be all close to the same measurement. If it has different measurements, there is usually a slotted adjustment where you can loosen the adjuster, make the correction, and tighten it up again.

Also, did you hit anything that may have bent a blade? Are the blades in good condition? If the blade tip on one end gets bent up, it will cause a sculpting effect to the lawn surface.

I hope that all helps!
Ed