Beware: The details will depend not only on jurisdiction, but also on the details of the parents, the parenting agreement and, of course, on the situation of the child. However, here are some general guidelines (mostly independent of jurisdiction):
Ideally, you should resolve the problem by non-legal mechanisms. However, you may have to resort to legal means if this fails. I would advocate a gradual escalation of your reaction:
- First, do not assume malice. Nicely ask parent A why the plan was not followed. Maybe it was a simple oversight, maybe there was an emergency? Try to find out, and decide whether the change was warranted.
- If there is no satisfactory answer, clearly remind A that the parenting plan is binding for everyone, and that it is important for both the child and the parents that they can rely on it. Stress that any last-minute changes must be discussed as soon as possible, even in emergencies. This should be done in writing, maybe even by registered mail.
- If the problem repeats, send a last letter indicating that you will seek legal remedy if the problem persists. This letter may work better when sent by a lawyer. A letter from your lawyer to A's lawyer (assuming you both have one) may also prompt A's lawyer to explain to A that they are hurting the child and themselves by violating the parenting plan.
- Finally, if all the above fails, go to court. You could ask for a change in the parenting agreement, maybe with less frequent changeovers, or with changeovers that are easier to arrange, or at an earlier time, such that a delay causes less problems. You could also ask for a formal permission to have the child fetched by the police or similar on subsequent violations (though that is a rather desperate option, and may not be available). If you reach this point, following the previous steps should give you a fighting chance to prevail in court, as you have demonstrated that you tried everything to make the agreement work. Courts generally take a dim view of people who violate an official agreement.
In Georgia specifically, like in most US states, violation of a court-ordered parenting agreement by one parent is a serious matter. The other parent can ask the court to hold the parent in contempt of court. The court can then order a number of consequences for these violations, such as awarding the other parent extra visits or monetary compensation, up to and including sending the parent to jail (this only happens in extreme cases).
The article Violation of Custody and Visitation Orders in Georgia gives a good overview.