The consolidation message shows up if the data in shown in the Snapshot Manager does not match the existing snapshots.
The main causes why a snapshot consolidation fails, are either low free disk space on the datastore (especially with thin provisioned virtual disks), or a backup application that uses Hot-Add, and did not release/unmount the VM's virtual disk from its backup server, or proxy. In the latter case, verify that no backup is active, and remove the mounted VM's virtual disk from the backup server/proxy (do NOT opt to delete it from the disk!).
André