How does the Presence of Mind Ult refund work?

Yesterday I had Karthus (who is my main champ) in an ARAM, and had Presence of Mind equipped. In that game mode, I often just go in and do as much damage as I can, and ult in the passive. After I died, I watched my ult cooldown and the information if the rune. Even when I got assists after dying, I didn't see my ult cd being decreased or the rune saying it has refunded more cooldown. I still got the refund then and now, but sometimes I felt like I didn't get it when I thought I should. Did I miss something on when the rune refunds CD, or is the rune bugged? Thanks in advance!