[Kevin Sevcik]

This is actually by design. Though it is odd that there isn't a mention of it in the manual. The center wheel is purposefully 1/4" lower than the outside wheels. Having the center wheel lower makes the robot "rock" to either the front or back side, so that only 4 wheels are touching at any time. This drastically shortens the wheelbase and makes the robot much much easier to turn than if all 6 wheels were even or if you were going with 4 wheel drive.