List of causes for Delay in Delivery are,

1. Unclear requirements- sometimes we find the unknown during the implementation.
2. Dependency – Code Review & Approval, API / UX availability for frontend from the backend team.
3. Improper or not in detail estimation.
4. Over estimation.
5. Unplanned leave or absence of team members.
6. Requirement change during clarification.
7. Delay in Development, delays in Testing and automation.
8. Proper Domain & functionality knowledge. PO & BA plays a major role.
9. Proper Junior and senior member ratio in the team.
10. Backup resource. They may be freshers or interns.
11. Onboarding & deboarding of team members due to performance issues.
12. Working remotely & isolation creates unclarity & delay.
13. Communication gap or late communication of the impediments or follow-ups.
14. Working at odd times when others are not working.
15. Time zone difference for review, clarification etc. As client senior members are in different locations.

If you know any other points then please do share them with others as a comment.