Judging from the description of the two, what does Chaucer think can cause a religious person to fail in his or her duty?
Which "two" are you asking about? There are several religious people in "The Canterbury Tales". The first two important ones mentioned are the Prioress and the monk. Both of these characters, though members of religious orders, are self-centered rather than God-centered. The Prioress, Chaucer tells us, was very concerned about her manners, her appearance, and her little dogs. She is not described as being a servant of God's or as tending to people. She is more concerned, in fact, with her pets than she is with people. She eats well and she is extremely mannerly as if ones manners spoke more about what type of person one was than one's devotion to God revealed. As a member of the clergy, her attentions should not be on manners, her appearance, and her pets. She should tend to people's needs. The same is true for the monk. He loves to hunt and spends a great deal of time at that. He is described as knowing his horses well and tending to them well. He has greyhounds that he uses in his hunting, in fact, he is described as sparing no expense to pursue the sport of hunting. He also dresses well and he, like the Prioress, is well-fed. As a member of the clergy, his attentions should not be on himself, which they clearly are. When a religious person becomes more interested in himself or herself, Chaucer sees this as the failure of the person. When he describes the Parson, later on in the General Prologue, it is clear that Chaucer sees his traits as the traits a person in a religious order should have. The Parson is pious and gives all that he has to the people of his parish. The Parson does not think of himself first like the Prioress and the monk do.