What do Rosencrantz and Guildenstern fail to reveal to Claudius in Hamlet?Rosencrantz and Guildenstern didn't find out the cause of Hamlet's madness.
In Act II, Claudius has told these two men to find out what is wrong with Hamlet -- why he is acting so strangely. The two were good friends of Hamlet and Claudius thinks that they will be able to find out what is wrong.
In Act II, Scene 2, the two men find out that Hamlet is only pretending to be crazy (or so he says). He tells them that when the wind is coming from the right direction, he can tell "a hawk from a handsaw."
The two men keep this from Claudius, just telling him they don't know why Hamlet is crazy.