The Seer Almine:
Because our Light family is as advanced as they are, the deeper complexities of the dynamics of many concepts you may have thought you understood, are being revealed. Forgiveness is certainly more intricate than generally thought. The reason for this is that individuals consist of layers of sub-personalities that are all involved in experiencing the events of your life unfolding.
Your adult may have understood through reasoning that no harm may have been intended, your nurturer may through compassionate understanding, embrace the offender, but the inner child may yet feel deeply wounded — in which case the offence cannot be forgotten. To answer your question: that is why your attempts to forgive someone, may be failing.