Final answer:
Friar Lawrence's plan in 'Romeo and Juliet' fails due to a critical message not reaching Romeo, leading to a series of tragic misunderstandings and the death of the two lovers.
Step-by-step explanation:
What prevents Friar Lawrence's plan from working in Romeo and Juliet is a series of unfortunate events and miscommunications. Initially, the plan for Juliet to fake her death using a sleeping potion was sound. However, the message informing Romeo of the plan never reached him. Grief-stricken by the news of Juliet's death, Romeo obtains poison and goes to Juliet's tomb. He drinks the poison just as Juliet awakens from her induced slumber. Upon finding Romeo dead, Juliet then takes her own life. These tragic events unfolded due to a critical letter not being delivered to Romeo.