I have quite a little bit of experience with bumble. Once a bird gets it, even if you remove the core and the bird heals, its just going to get it again. If your birds are over 14 ounces nothing you can do will stop some of them getting bumble. Their bodies can only physically handle being a certain size. In the last 30 years the average size of a jumbo more than doubled. Their bone and tissue structure is only capable of supporting so much weight and people have pushed them past that point.