Good to know: The creator of an item retains ownership of that item unless that ownership role is changed by an Administrator or a User with permissions to change ownership of a shared item or folder.
Steps to troubleshoot
A System Administrator or User with the appropriate permissions will need to go through the following troubleshooting steps:
1. Place a check next to the name of the Shared Data Extension, and then select 'Change owner' in the contextual menu to take ownership of the Shared Data Extension.
2. Import permissions enabled at the granular level by following these steps:
1. Go to Admin | My Users.
2. Select the User's name.
3. Click Manage Roles | Permissions | Email | Subscribers | Data Extension | Import.
3. Any Data Extensions being shared must exist in the main Shared Data Extensions folder or a sub-folder. You can change permissions on Data Extensions like with other shared items.
4. The owner of the Data Extension must move it from the Data Extensions folder to the Shared Data Extensions folder in order to share it.