Answer:
In Scene 3 of Act 3, Claudius was praying which disabled Hamlet to kill him while in Scene 4 he was not praying.
Step-by-step explanation:
In Act 3, Scene 3, of the play Hamlet, when Hamlet goes to the chamber of king Claudius to kill him, he did not kill him because Claudius was praying.
Hamlet thought that if he will kill Claudius while he was praying, he will receive heaven because he repented for his sins. And also that Claudius did not gave hamlet's father this chance, because Claudius killed old King Hamlet in the state of sleep when he could not pray and repent for his sins.
In Scene 4 of Act 3, Hamlet was asked to summon by Gertrude in her chamber, Polonius was hiding behind the curtain and eavesdropping the conversation between Hamlet and Gertrude. Hamlet mistaken Polonius to be Claudius kills him. He was able to take action in Scene 4 because he thought that Claudius is not in a position of repentance.