Check the tire pressure. Also, if a wheel on the deck are pushed up (bent bracket) it may cause your problem. That's all I can think off.
My old Craftman isn't level, and I've tried (not very hard) to fix it. It's not a priority, so I just cut in a checker pattern or go over the lawn a second time at top speed. My yard looks like the outfield of a baseball field with the checkered pattern...and I get to use top speed on the mower while pretenting I'm at the race track
