Yeah, this one's common. It's usually the deck height being uneven or the blade deck itself is out of level. The height adjusting lever (part 32) connects to a mechanism on each wheel.
First, put the mower on a flat surface. Check that all four wheels are set to the same height setting. The adjustment mechanism can slip on one side.
If the heights match, the wheel assemblies (part 10 or 37) might be damaged. A bent axle or worn bushing can tilt the deck. The front axle set (part 38) is a specific part. Also check the rear cover (part 20) for damage that's warping the deck.
Less likely, but if the blade was replaced incorrectly, it might not be centered on the spindle, reducing its cutting width. Make sure the blade is installed with the correct washers and bolts (part 29, 30).
A quick fix is to measure the blade tip height at all four corners of the deck. They should be within 1/4 inch. If not, you've found the problem.