The fact that you have already gone through a divorce that dissolved your marriage does not mean that problems cannot and do not arise even AFTER the divorce.  Typical problems relate to the exchange of property pursuant to the parties' settlement agreement and/or divorce decree, payment of child support and expenses, violations of the child custody and visitation arrangements, etc.  When the parties' cannot agree on a resolution of a post-divorce issue, the only option is usually to take the other person back to court to have the judge resolve it.