It's somehow strange. I checked my settings and yes, it's enabled, timer set to 5 seconds.
When I finish a custom game or VS A.I. and gain a level upgrade, then - and only then - a toast notification is displayed. While playing campaign, none is displayed.
And like I said, it occurs only on OS X. This problem seems to exist since the patch 1.5.xxx
Is there no fix? It's annoying because I don't know when I earn an achievement while playing. I have to navigate to the Achievements screen to see what I got and what not.
Come on...there has to be a fix....