Milestones in project management can often be missed for a variety of reasons, including:

1. **Poor Planning**: Inadequate initial planning can lead to unrealistic timelines and unclear objectives, causing milestones to be unattainable.
2. **Resource Availability**: Lack of necessary resources (such as manpower, materials, or equipment) can hinder progress and lead to missed milestones.
3. **Scope Creep**: Uncontrolled changes or continuous growth in project scope without corresponding adjustments in time or resources can delay deliverables.