I've used EasyDriver boards before to control steppers, and when a motor is in use but not rotating, the driver board holds the motor in place so it can't be easily turned by outside forces.
With the motor shield I'm experiencing somewhat different behavior. When neither motor is moving, one is held in place while the other can be rotated freely. When they're moving, there's no problem, but when they stop, one of them (it alternates which one) can keep moving.
Is this a software problem (e.g. a bug in the sketch), or a hardware problem (e.g. the driver IC is defective), or a fundamental limitation of that motor driver shield? And the obvious follow-up: how can I fix this problem?