In order to merge accounts, you have to have certain manager permissions to do so. Please contact Matrix Support to see if you have the proper permissions.
If you have the proper permissions to merge accounts, you can do the merging/maintenance by following the instructions on the Local Account Maintenance - Overview Knowledgebase Article.
If you do not have the proper permissions and are not permitted to have the permissions (based on "corporate" decision) please send your merge request to Matrix Support.
If you have Local Account Maintenance abilities, you will be able to merge accounts that are only in your local market. If there is an account that you wish to merge and it is not an option in your merge list, then this account is also in another market and you will not be able to merge the account. However, that doesn't mean that the account cannot be merged. It just means that you cannot merge the account yourself. We at Matrix might be able to merge the account, so again, please send the merge request to Matrix Support.