Is it normal for the carriage wheels to ride at different heights on the black track? Some of the wheels seem to be low on the track and then the wheels right behind it are very high on the black track. Do these wheels ever get adjusted by loosening or tightening the bolt that holds them on? Also, all eight wheels are not on the track sometimes. When I move from back to front all of a sudden it shifts from being on some wheels to others as the machine applies weight. Is this right? I'm so frustrated I'd send this machine back in a minute if I had the option.