This is such a loaded question and so much depends on circumstance.
It is important for all parents to know that all kids, at some point refuse something.
It could be refusing their dinner, sleep, homework, their extra-curricular activity, going to the dentist… anything. There isn’t always something nefarious to a child’s refusal to do something either. Sometimes it is fatigue, it could be a different preferred activity at the time, it could be seeking to avoid (dentist), or it can be a teenage child seeking to be more independent and hang out with their friends, or wanting a part-time job, which can be developmentally appropriate.
Typically though, we do seek to hold kids accountable to the parenting schedule just as we would about seeing the dentist or going to school. Some things tend to be non-negotiable.
Between two parents who mostly get along, and depending on the situation, the child may be allowed to opt out, they may be allowed to defer, or they may be required to follow through. The parents would chat about the situation and determine their course of action.
Between separated parents who don’t get along and/or don’t communicate well, having a child who doesn’t want to follow the parenting plan can be like throwing a grenade into the situation. Everything may blow up.
The go-to concerns for parents in these situations is avoidance of some sort of abuse or alternately, the child is being either withheld or influenced negatively towards that parent.
Parents are cautioned, before thinking the worst, it is important to understand what is beneath a child’s resistance to following through with the parenting plan. It is important to consider the age of the child; and it is important to see if this is a one-time issue or part of a trend.
Parents are also cautioned about making assumptions, no matter how conflicted the relationship between the parents. To the degree possible, seek to have a reasonable discussion with the other parent. If you deem that impossible, then you will need to use a third party to facilitate communication. The key is to seek understanding, communication, and to address any concern arising. Even if you feel it unjustified, it still must be addressed.
If the matter falls to your lawyer to facilitate the communication, be certain that the communications remain inquisitive and not accusatorial. Communications are to be directed to understanding and resolving issues as best as possible.