The key with introducing anything into a rabbits diet, whether it is vegetables or pellets, is to go slowly.
Give a small amount of the veggie you are trying to introduce, and if no reaction is seen, slowly increase the amount you give your bunny daily over the period of a couple of weeks. Then, once the veggie is established, you can start adding another veggie. Keep slowly adding in veggies until your bunny is eating a wide variety.
By adding a veggie one at a time to a bunny's diet, you should be able to pin point if a particular vegetable disagrees with your bunny's tummy. Some rabbits can eat pretty much any vegetable with out a reaction, some rabbits get gas from some vegetables but are fine with others, and some rabbits have super sensitive stomaches.
Generally, most people start out with leafy greens like romaine lettuce, green leaf/red leaf lettuce, parsley, cilantro, carrot tops and work in some other veggies like broccoli, bok choy, and peppers.
-Dawn