First of all check the Connections documents and check the replication setting in these Documentes. - I take it this is the only DB that is not using the cluster replicator? This would mean that replication and cluster replication would not be the issue.

Second of all, try to remove the cluster then re-create it .- I would not do this if cluster replication is working else where….. but Right Click the DB in the files TAB and click cluster – Then mark it in service > and the right click > cluster and then enable cluster replication.

Things to check – the replica ID is correct for the DB
Type Show cluster on the admin console to see the cluster informaiton for the server – if this is wrong then add and remove from cluster.
If you need to delete one of the DB’s and replica a new DB from the other server. This may be a replication issue (note you could clear the DB replciation history as well)