[mmaunu]

It sounds like your wheels aren't spinning in the correct directions. We had similar problems a couple of years ago. Have you inverted the motors on the left side (or right side) like Joe mentioned? This ended up being the cause of our woes; we hadn't inverted any of our motors.

First, are your wheels mounted correctly? When you look down on the robot from above, do the front rollers want to roll in towards each other when the robot moves forward? Do the back rollers want to roll away from each other? Here is a top-down view:

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Put your robot up on blocks and look at the way the wheels spin (again, look from above).
1) When you direct the robot to go forward, all wheels should spin forward.
2) When you direct the robot to go backwards, all wheels should spin backwards.
3) When you direct the robot to strafe right: the right front wheel should spin towards the back of the robot, the right back wheel should spin towards the front, the left front wheel should spin forwards (towards the front), and the left back wheel should spin backwards (towards the front).
4) Test again with strafe left: left front should spin towards the rear, left rear towards the front, right front towards the front, and right rear towards the back. (The wheels diagonally across from each other should turn in the same relative direction.)
5) When you rotate clockwise, the right wheels should rotate towards the back of the robot and the left wheels should rotate towards the back of the robot. (The wheels on the same side of the robot should turn in the same direction...and each side of the robot should turn in opposite directions.)
6) When you rotate counter-clockwise, the right wheels should rotate towards the front and the left wheels should rotate towards the back.

I am not sitting in front of a robot so take those directions with a tiny grain of salt. Somebody feel free to correct me.

Again, it sounds like a symptom that we had to track down before. Try inverting the left motors but not the right (or vice-versa). This can be done in code (Java and C++ have method calls on the speed controllers and LabView allows you to set this property as well).

I hope that this helps. Mecanum is fun to drive ~(_8^()