I have learned from sad experience that not everyone values communication equally.
Never mind differences in values, sometimes our timing is not in sync. While you may benefit from having a conversation and trying to resolve conflict or find closure, it is reasonable to assume that others, at times, will weigh the benefits and risks of talking and determine that moving on, without talking, is less risky and more beneficial.
This can be incredibly difficult to accept when we find ourselves on the side of conflict that wants to work things out or find closure.
That said, there are a few things you can do.