What could be possible reasons of OSPF neighborship not coming up?

There can be multiple reasons regarding this issue .

1. You have not enabled ospf on correct interface .

2. Interface on which ospf is enabled is shutdown or line protocol is not up.

3. Ip address on both routers are from diffrent subnet.

4. Acl is applied on some interface denying ospf packets.

5. Stub flag is not same both side.

6. MTU at both ends is diffrent.

7. Area number may be diffrent .

many more issues can be there.