fredley,
Thats happend to me as well. All the times it has occurred it was because the sound card was temporarily unavailiable ( being used / locked by another app ) or, In one instance, I had lost firewire connection to my Echo Audiofire Pre8 and the way just stayed busy untill I happen to look at the rack and realized that the sound card had lost connection. When I re-cycled the sound card, the wave screen redraw was immediate!
YMMV but that did it for me...