She needs a much more varied diet than just collard greens! She also should have plenty of hay - alfala until she is full grown. Right now she's not getting a balanced diet. Try adding parsley (Miss Hoppy says the flat "Italian" pasley is tastier than the curly stuff), dandelion greens and flowers if you have dandelions in your yard (you can buy the greens at the store) stuff like dill, Romaine lettuce, carrots and carrot tops - see
http://www.rabbit.org/care/veggies.html for a good list.