Rabbit question!

I was speaking with a guy who asked why his rabbits were not breeding like rabbits. I asked about his setup and he said:

The rabbits are a brother and two sisters, three or four years old, that always have full feeders and can eat as much as they want. He said they mate and the male falls over, but the females have never gotten pregnant.

I think the rabbits are probably too fat and maybe getting too old, but what do I know, I raise chickens!

Should I tell him to put them on a diet and try again or just cook up that batch and start with unrelated critters? Any other advice to pass along?