[Greg McKaskle]

Sometimes it helps to make a mnemonic ...
"O from below make Mecanum go"

To think it through, if the rollers are set up to be an X from below, by hand, you will be able to spin the robot about the center of the X with almost no resistance. The problem with that is, the robot now has no resistive force to propel itself about that axis. Reversed wheels will still allow it to move forward, backward, left, and right, but not to spin. It may also help to draw, or imagine, the friction force vectors aligned with the roller axis where it touches the ground.

Greg McKaskle