The effectiveness of the brake is based on engine RPM. Can you describe what you were doing, how fast you were going, what gear you were in, what gear the coach automatically selects when the switch is pressed?
Did you by chance, press the low option first, not feel a thing, then press the high option? Is that what you're saying? Depending on my speed the low selection can have little effect for me in some cases, then I press the high button.
Mike