Unless you know exactly what relay you should connect to fetch this event in amethyst you cant do nothing.
I usually open the event in noStrudel and there it shows an option with a relay list to try to fetch the event if the event is not found 

 I long press the missing note, copy the note ID, then paste it to a new note, x it out so it goes to drafts, then check drafts. I'd say 3/5 times, that works to find the note without leaving amethyst. Its a janky work around