Why doesnt my rabbit feed one of it's bunny?

Your doe isn't ignoring the baby. The kits are all together in a nest in a nest box, right? They all snuggle together for warmth that way and all get a chance to eat at the same time, when she stands over them for them to nurse. If one is not getting fed or seems to not grow as fast as the others, it's probably weaker than the rest. Since does only nurse the kits once or twice a day, you can remove the stronger kits (to a warm safe place) for several hours without a problem and leave the weaker one in by itself, or just one or two litter mates, so it stays warm enough and gets a better chance to eat. Do not give it any supplemental feeding; this can cause more harm than good, disturbing its digestion.

Remember that the mortality rate for baby rabbits is high and they don't all live. That's one of the reasons rabbits have so many litters of so many babies.

Are you sure it's her bunny? Maybe your naughty brother put a stuffed bunny to trick you....