This phenomenon is very common. The point is that there very often are problems initially for one reason or another but as time passes these problems do more often than not resolve themselves. It is important to understand that matters relating to children can be brought before a court at any time and nothing is final because the courts will always decide any particular issue on the basis of what is in the particular child's best interests at that particular time. The Statement of Arrangements for Children, therefore, is not a binding document in any way and more often than not it is merely a statement of intentions. Even if those intentions are not entirely acceptable to the respondent it can be sensible to wait to see how things develop before asking a court to intervene.
Matters can, and do, change over time.
It is worth bearing the foregoing in mind because the number of cases coming before the courts involving children when a relationship breaks down is relatively quite small in relation to the number of children involved. Most people do work out more or less satisfactory arrangements without any court intervention and the paragraphs above describe how this happens in the context of divorce proceedings. Nevertheless, there are some intractable cases where it is absolutely necessary to ask the courts to decide. One parent may steadfastly refuse to let the other parent see the childen at all, for example. Or there may be threats to take the child to a foreign country etc.