How to feed up an underweight rabbit?

I have got a rabbit who i think is underweight...she is a dwarf lop eared but is quite big - you can feel her back bone and is bony on her rear end. I feed her a complete rabbit care mix and she also gets dandelions and veg. What should i be feeding her? She has access to a large patio to run around as well as the security of a cage. Any suggestions?