The possible causes are as follows:

1.An interface is not Up.

2.OSPF is not enabled on an interface where it is needed.
3.An interface is configured to work in silent mode.
4.The OSPF Hello packet is filtered out by an inbound ACL.
5.The interface masks are mismatched.
6.The Hello timer values are mismatched.
7.The authentication types or authentication keys are mismatched.
8.Area IDs are inconsistent.
9.Area flags are inconsistent.

informative post. nice

good troubleshooting.

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helpful info

useful document, thanks