What causes this kind of conversation loop?

I had to give a speaking test to first year students at my one JHS. For the past 4 months the JTE has been drilling them with small talk and how to give a reaction.

The student were giving a random paper with my interests on it. For example, anime, books, sports. The conversation would go like
S: Oh, you like books.
ALT: Yes, that's right. I do.
S: What books do you like?
ALT: I like fantasy.

That would be a B grade. An A would be any extra question after. Out of the 4 classes only one class(JTEs homeroom) did exceptional. The rest performed low or got B.

Now my question is what causes students do give these conversation loops. For example, I got a lot of Oh, you like sports. Followed by do you like sports?

I don't understand why it's hard for a student to substitute one word. For example, they can say What book do you like? Oh, I like Lord of the Rings. They can't follow up with something like What character do you like.

I talked about this with my JTE. I wondered if it is because they are still young they don't know how to even have a conversation in Japanese. The JTE said no but she didn't know why. Also, many of the students wanted to derail the conversation into a topic about them which was an instant C.

Sorry for the long roundable question. I'm interesting in what others have to say.

by AdUnfair558