You may find that wiggling the mouse pointer around the screen “unsticks” the playback line, and you may also find that switching modes, e.g. to Engrave mode and back again, either by clicking on the buttons in the toolbar or using the shortcuts Command-3 and Command-2, get things moving again.
I’m sure we will eventually manage to get to the bottom of this, but so far we have been unable to. The problem is difficult to reproduce on demand and is therefore difficult to debug.