mecanum wheels on frame

[7-11number1]

This is team 1711 and our robot is having "minor" technical difficulties. Were using 8'' diameter mecanum wheels and it has issues when it comes to driving around. The robot handles forward, backwards, and strafing side to side perfectly well. But the wheels lock up when attempting a turn or circling around on its center of axis.
The wheels are position with the left and right wheels 20'' apart and the front and back being 19''. I know a perfect square is recommended for mecanums but would a small difference like an inch cause the wheels to lock up when attempting turns? We need an experts advice or some links that provide good information on how to properly mount and place mecanum wheels.

http://www.flickr.com/photos/23220170@N07/
(ignore pic with tread and mecanum wheels)