Milestones in project management are often not met due to a variety of reasons, including:

1. **Poor Planning**: Inadequate or unrealistic planning can lead to unclear objectives and timelines, making it difficult to meet set milestones.

2. **Scope Creep**: When project requirements change or expand without proper control, it can lead to delays in meeting milestones.

3. **Resource Allocation**: Insufficient resources, whether in terms of manpower, budget, or equipment, can hinder progress and affect the ability to meet milestones.

4. **Lack of Communication**: Ineffective communication among team members can lead to misunderstandings, missed deadlines, and ultimately the inability to meet project milestones.

5. **Risk Management**: Failure to identify and mitigate risks in advance can result in unforeseen obstacles that delay progress.

6. **Team Dynamics**: Poor team collaboration or conflicts within the team can disrupt workflow and lead to missed milestones.

7. **External Factors**: Uncontrollable external factors, such as regulatory changes or market shifts, can impact project timelines and milestone achievement.

Addressing these issues through careful planning, robust communication, and effective risk management can help increase the likelihood of meeting project milestones. 

Would you like to explore more specific reasons related to a particular type of project or industry?