At the age your puppy is, the behavior that you describe is 100% as it would be expected to be. Your dog is a B-A-B-Y. At this stage of life, just be happy that she chases and picks the bumper up -- and at least brings it part way to you some of the time. Use the "hallway method" and the check cord method as well, but DO NOT require perfect compliance. Enforced obedience shuldn't even begin till the dog is 5-6 months old. And Force Fetch cannot be done until teh dog has all it's adult teeth in (minimum 6 months). At your pup's age, it should all be fun and games.