He was quarantined.
Friar John got put in quarantine and the letter never got delivered.
Well, that is a complicated question, because no letter like that is delivered: There was a letter to Romeo talking about the false-death plan, but it was never delivered. Friar Lawrence wrote that letter and Friar John was supposed to deliver it, but was unable to due to a quarantine. Balthasar is the one who tells Romeo that Juliet has died (although she has not... this is the false-death that she will awake from), but he just tells him; he doesn't deliver a letter.
A letter was sent from Friar Lawrence to Romeo who had been banished. Romeo had not intercepted this letter, and after having knowledge that Juliet was 'dead' he rushed to the tomb.
There was a plague going around and no one was allowed to enter the city or Mantua
The excuse Juliet uses is that she is going to Friar Laurence's cell for shrift or confession.
Friar John got put in quarantine and the letter never got delivered.
Well, that is a complicated question, because no letter like that is delivered: There was a letter to Romeo talking about the false-death plan, but it was never delivered. Friar Lawrence wrote that letter and Friar John was supposed to deliver it, but was unable to due to a quarantine. Balthasar is the one who tells Romeo that Juliet has died (although she has not... this is the false-death that she will awake from), but he just tells him; he doesn't deliver a letter.
Friar Lawrence requests a crowbar to open Juliet's tomb and a letter to write to Romeo explaining the situation with Juliet.
it wasn't delivered because Friar John, who was supposed to deliver it was quarantined because of the plague
A letter was sent from Friar Lawrence to Romeo who had been banished. Romeo had not intercepted this letter, and after having knowledge that Juliet was 'dead' he rushed to the tomb.
The letter is not delivered to Romeo because it gets wrongly intercepted by Friar Laurence's messenger. This is a conflict related to miscommunication and the consequences of a failed message delivery, which adds to the tragic events in the play.
There was a plague going around and no one was allowed to enter the city or Mantua
The excuse Juliet uses is that she is going to Friar Laurence's cell for shrift or confession.
Friar Laurence's letter is not delivered to Romeo because Friar John was kept in quarantine since there was a plague in his house. Friar John wanted to go to Mantua himself but he couldn't. Once Friar Laurence hears this he says: "Now must I to the monument alone. Within these three hours will fair Juliet wake. She will beshrew me much that Romeo Hath had no notice of these accidents; But I will write again to Mantua, And keep her in my cell till Romeo come." His plan is to go to the tomb, get Juliet then write a second letter. But Friar Laurence's second letter is never sent to Romeo because Romeo's servant Balthazar has already told him Juliet is dead. Romeo arrives at the tomb before Friar Laurence can even write the second letter, which results in Romeo killing himself, which results in Juliet killing herself.
What letters are there in Romeo and Juliet? Tybalt sends a letter to Romeo's house, which Romeo doesn't get because he doesn't go home. Friar Lawrence sends a letter to Romeo via Friar John which doesn't get delivered. Since the letters are not delivered, they cause nothing. If they are dramatic images of anything, they are images of miscommunication.
Friar Lawrence entrusts Friar John with the important letter to Romeo, instructing him to deliver it to Romeo in Mantua. However, Friar John is unable to get the letter to Romeo in time due to being quarantined.
He sends a friend and fellow friar, Friar John, with a letter.