Database is up to date, but some sites are not completely upgraded

Problem: Getting database status as "Database is up to date, but some sites are not completely upgraded" from "Review Database Status" page in Central Administration,

Root Cause: This simple means: SharePoint Products Configuration wizard is unsuccessful in upgrading the specific database.

Solution:

• Go to SharePoint Central Administration >> Upgrade and Migration >> Review database status >> Get the list of Databases that need upgrade.
• Fix database upgrade status with below PowerShell cmdlet: Upgrade-SPContentDatabase

Fix database upgrade status with PowerShell cmdlet: Upgrade-SPContentDatabase

Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue

#Set this variable value accordingly 
$DatabaseName = "content_database_name_here"

#Get the Database ID
$DatabaseID = (Get-SPContentDatabase -identity $DatabaseName).ID

#Update Content Database
Upgrade-SPContentDatabase -id $DatabaseID -Confirm:$false

Once the process completes, Go back to the "Manage Databases Upgrade Status"page in Central Admin and check the status, It should be "No action required."

Upgrade All content Databases:
Rather doing it one by one, lets upgrade all content databases in a single stretch.

$WebAppURL= "http://intranet.crescent.com"

#Get all content databases of the particular web application
$ContentDBColl = (Get-SPWebApplication -Identity $WebAppURL).ContentDatabases

foreach ($contentDB in $ContentDBColl)
{
 #Updade each content database
 Upgrade-SPContentDatabase -id $contentDB.Id -Confirm:$false
}