He seems to have developed some sort of resistance to the procedure.

Clem: What, Joel, What are you doing?
Joel: We gotta go. Go on. Please, please, go. Please go!

Stan: You can run, but you can't hide.
Joel: Clem, come on! Please.

Stan: That's strange. He's in a memory that we've already erased.
Howard: Oh, well, at least we know where he is. He's back on track, right?

Joel: Come on, Clem.
Howard: He seems to have developed some sort of resistance to the procedure.
Clem: I don't want to run anymore.
Joel: Shit. Come on, come on. Clem, come on!

Eternal Sunshine of the Spotless Mind (2004) by Michel Gondry