I second the porridge... used to keep my elderly bun's weight up in winter.
I just used readybrek, and used warm water to make it. She adored it, scoffed it down! I used to give her a whole bowlful and supply unlimited other food too. I'd either put honey or a teaspoon of sugar on top, and sometimes added raisins. Obviously make sure its luke-warm when you give it to her.
I cant think what else to do if she's still eating the same amount as before. My rabbit book said rabbits stop eating if they are thirsty - so maybe feed more wet foods and fresh foods?
EDIT: sorry just read your repy to the one above, you mustve written it whilst i was writing, ooops/