Might be worth reporting it to Network Rail if it's doing it frequently, if it's not just happening immediately after the barriers are raised. I don't know the exact figures off-hand, but they're usually set up so that the motor starts driving the barrier back up at a greater angle (from horizontal) than that at which detection is lost - i.e. they should sort themselves out before detection's lost. Should also indicate failed in the box too, if they do lose detection.