There are many issues during a divorce that are interpersonal in nature but become legal matters as a part of the process. Because of this inherent part of divorce, it provokes emotional responses from the splitting spouses -- especially when the topic at hand is about their children. Every divorcing parent hopes that they can make joint custody work, but sometimes that isn't an option, or the agreement just isn't followed by one of the spouses, causing even more tension after the divorce.
When a parent doesn't follow the child custody agreement or is deliberately making things difficult for his or her ex-spouse, this is called "interference." Interference can occur in a number of ways. Maybe one spouse is constantly very late to meet-ups to assume custody of the child; maybe one spouse refuses to pay child support; maybe one spouse fails to pass custody off to another spouse because they feel they don't get enough time with their kids.
To a certain degree, you may have sympathy for the offending spouse in these hypothetical situations. There are real emotional factors at play in all of these situations. It's just that they are dealing with their emotions in the wrong way. Interfering with the child custody agreement can lead to legal penalties and the permanent alteration of the custody arrangement.
If you are dealing with a difficult spouse who is not obeying the child custody agreement, then you should talk with a lawyer. There are ways to modify your agreement to alleviate the situation at hand.