What are you driving? Easamatic brakes? Has there been any work done recently to the brakes? Is this a new issue, or one that has been progressively getting worse? Fluid level okay? Any noticeable leaks? Have you had to add brake fluid recently?
It could be that they are working while in motion because the rotation of the wheels add mechanical force when braking that doesn't happen at a stand still.
Fluid level would be my first check, and then bleeding the brakes after that.