- A network partition is preventing all members of the replica set from communicating with each other.
- There are not enough voting members in the replica set to keep a primary. A simple majority is required to have a primary. This can happen if too many hosts are down (or not responding) to replica set heartbeats.
- There is not an electable member when the primary goes down.
- Ensure a majority of votes are available in the replica set. If hosts have been shut down permanently, ensure they are removed from the replica set configuration.
- Use basic ping tests from one host to another to ensure they can communicate with each other
- Make sure there is more than one data bearing member with a priority greater than 0.