There are unfortunately many different ways in which a parenting schedule can be violated. How to deal with each type of violation often depends upon the extent of the violation, the frequency, the reason given, whether an alternative schedule can be agreed upon, is a make-up possible and ultimately whether the court needs to be involved. Either way it is a good idea to keep a calendar with date & time of the problems as well as copies of all written communication whether by text or email.
A parent that is repeatedly late either in providing access to the children (kids never "ready" on time, "still having breakfast", "got up late") or late return ("traffic", "the kids were having fun" "the kids were still eating") can be dealt with in several ways. One approach is to get extra or less time as a result of the late turnover or late return. This is not always easy to implement but it is one approach. Another approach is to modify the schedule by 15 minutes to make it work better for both parents. Of course filing a violation petition is the ultimate option to enforce the schedule but an assessment should be made if the time/cost involved is necessary. For minor time violations it may not be the best move but for a half hour or longer it will likely be necessary.
A parent who denies the entire visit without good cause is a potentially bigger problem. Again it will depend on the reason. If the reason is the child is ill to the extent that an exchange does not make sense then sometimes that is the case (child vomits or runs fever vs has a slight cold or sore throat or slight temperature). Sometimes a make-up can be scheduled but I have seen parents who also say there is no make-up for a sick day because it was not a "fun" day for me. However, I have also seen parents who provide the make-up. It is less likely that the court will do much if the child was genuinely ill and the illness could not be attended to by the other parent. If the child has a broken leg, there may be a few days of missed visits until the child is more mobile. Perhaps a visit can occur in the home where the child is. Many parenting agreements will have this provision.
A parent who is a no show and thereby does not permit pick-up is an even bigger problem as is a parent who will not return the child. Again, the first question is the reason and the second question is the frequency and intent of the parent not turning over the children. If this is an ongoing problem a court application will be necessary to determine the violation and the consequences. A parent who says "the child was busy or didn't want to go" is not providing a good excuse but if the child is 16 years old, then it may be true even if not acceptable. On the other hand a 5 year old child should be produced or returned. Of course there are exceptions to every situation. If a parent learns of domestic violence, neglect or abuse then the child should not be provided but an immediate court application should be made.
Ultimately the goal is to try and resolve these problems between the parents with some flexibility provided it is not a reduction in time. When that cannot be accomplished or for ongoing and repeated problems, then there is no alternative to filing a petition to either modify or enforce parenting time and possibly modify custody.
Consultation with an experienced matrimonial attorney can often lead to quicker results as the attorneys can file petitions faster and can also suggest modifications or work arounds to existing problems based on their vast experience with difficult situations.