Example is one thing. How do you and your spouse behave? How do you behave toward your parents? Does this child have other outside influences..TV, not enough catechism, bad playmates etc. those are all things to rule out.
Once you have ruled out or corrected external factors, consider that every child, depending on temperament is inclined to particular sins or faults. In that case, you will have to work out the most effective way to help the child root out those faults by rewards and punishment. You may not always be 100% successful, that is the nature of the effects of original sin.