Many of Bud’s weaknesses are actually external (such as poverty and homelessness); however, Bud has two main internal weaknesses as well: his inability to show emotion and his rules.
Largely because of his external weaknesses, Bud has learned to keep his emotions inside. We know this because his “eyes don’t cry no more.” Even when he is forced to part from Bugs, his “blood brother,” Bud watches, dry-eyed. Bud remains stoic through all of his trials until he actually meets Herman E. Calloway and finds the jazz band with which Bud belongs. At this point, “some rusty old valve squeaked open in me” and Bud begins to cry and show emotion. Finally, Bud shows real compassion for other characters, such as when het pats the “grumpy” old Calloway on the back when Calloway learns the fate of his daughter.
Bud’s firm and fast “rules” can also be seen as a weakness. For example, consider Bud’s rule that he calls "Rule 328":
Once you’ve made up your mind to do something, get to it before you have the chance to talk yourself out of it.
This rule comes up when Bud is locked in the Amos’ shed. Bud convinces himself he has found a “vampire bat” and decides to swing a rake to defeat it. Unfortunately, Bud’s rule causes him to disturb what is really a hornet’s nest and leads to many painful stings.