Scrubbing alt - space not working

Scrubbing with alt-space does not work. Scrubbing one voice works perfectly (shift - alt - space). But scrubbing alt-space only, shows signs of life when I hold down alt and shortly ‘hit’ space - then the loudspeaker flashes briefly and the sound is played for half a second. That’s all. But when I hold down both keys as recommended, nothing happens. Do I need to change anything?