Great post. Another issue is why B doesn't believe Y in spite of believing X and in spite of A believing that X implies Y. Some mechanisms:

a) B rejects that X implies Y, for reasons that are good or bad, or somewhere in between. (Last case: reasonable disagreement.)

b) B hasn't even considered whether X implies Y. (Is not logically omniscient.)

c) Y only follows from X given some additional premises Z, which B either rejects (for reasons that are good or bad or somehwere in between) or hasn't entertained. (What Tyrrell McAllister wrote.)

d) B is confused over the meaning of X, and hence is confused over what X implies. (The dialect case.)