I have never heard the sentence "Why hasn't the summons happened?" before. Do be completely honest I don't even know what that means. Could "Por que a convocação não aconteceu?" be translated as, "Why didn't the call happen?" or "Why hasn't the call happened?" I'm just trying to put it together through context. Thanks!
There seems to be little consistency in the knowledge of whoever has made the exercises in this lesson. At one point, "convocação" = call, where "summons" was marked wrong, and now "call" is not accepted, but instead "summons". Learning a new language is really hard already without such inconsistencies. /rant over
"Why has the meeting not taken place?" or "Why has the call not gone out?" Are we talking about convocation as an invitation or as the assembly itself (the English word can mean either)? The English Duo gives is discordant to an English speaker. Calls or summonses do not "happen" or "take place."