While there is no universal definition of what makes a parent a failure, a compelling case can be made for Francis Obi's failure in this area. It is also easy to argue that he is a failure as a husband and as a breadwinner.
Soon after Adah marries Francis and moves in with his family, it becomes apparent that she is to be the only one in the household to be working. Given that Adah quickly becomes pregnant with their first child, Titi, Francis's failure to earn an income could arguably make him a failed parent right from the outset.
Once the family is reunited in London, Adah once again goes out to work while Francis stays home, but he does not want to take care of the children, which compels Adah to leave their children in an unsafe environment with a "daily-minder" named Trudy.
After the difficult birth of her third child, Adah cannot work for a period, and Francis is forced to get a job. Despite having been supported for years by his wife, he fails as a parent by feeling the need to complain bitterly about having to support his family.
When Adah tries to go on contraception, Francis finds out and fails his children once again by beating and humiliating their mother. Given that he beats Adah when the mood strikes him throughout the novel, as well as throwing regular verbal abuse her way, he fails his children by failing to create a nurturing, stable family life.
Later, after Adah decides to leave Francis, he fails his children once again by denying that he is their father and burning their documentation.
In a nutshell, Francis is a failure at every aspect of being a husband and father.