I have seen this problem on 4 different computers so far, and someone else in this forum complained about it too. So I wonder if you really followed my description to reproduce this error. Without two monitor operation high quality zoom always works.
Ok, I have to redo my statement. Working on a new notebook (one display) I just found out that the HQ zoom does not work in the full screen mode (F11). So this has nothing to do with one or two monitors. It's this special view mode that disables (lacks) the HQ function.
High quality zoom works fine in fullscreen. However, I have noticed an oddity. If you zoom in first and THEN go to fullscreen, the quality of the zoom will drop dramatically. HOWEVER.... If you zoom out & then back in again while still in fullscreen mode, the high quality zoom is restored. There's some sort of glitch in the transition from regular view to fullscreen mode.