This is more of a Japanese language question, but since it's based on goofy anime scene, I thought I'd ask here.
The scene is from episode 88 of Gintama:
Gin asks Kagura to go to the ladies' room with the other women on a group date and play up his good looks. Paraphrased into English except for the word "cute," it goes like this:
Gin: "Remember what I told you to say?"
Kagura: "What do you think of the boys? My favorite is that silver-haired guy. He's ka-wa-i-i!"
Gin: "Not like that! Ka-wa-wi-i!"
Gin: "No! Ka-wa-" (here he chokes on his coffee)
I love this scene, but it's always bugged me that I can't tell what fault Gin is finding with Kagura's delivery that second time. Their pronunciations sound identical to me, except that maybe Kagura might be adding an n sound to the end of hers. If so, though, I'd expect Gin to tell her to drop it rather than say the word from the beginning again.
The only other possibility I can think of is that Kagura isn't saying ka-wa-wi-i very enthusiastically; in fact, she looks bored. But then, Gin says it in a pretty flat voice too.
So is there some sound difference that's inaudible to this non-native speaker? Or is it something else?