After Mount-SpcontentDatabase,having Database is up to date, but some sites are not completely upgraded. On /_admin/DatabaseStatus.aspx. When you click on the /_admin/UpgradeStatus.aspx it shows the status Succeeded
First of all, identify what site collection was not upgraded.
Run: stsadm.exe -o localupgradestatus > upgradestatus.txt
PSConfig.exe -cmd upgrade -inplace b2b -force -cmd applicationcontent -install -cmd installfeatures (Run from one of the WFE server)
As you see below, there is no site collection that needs an upgrade.
If you have any, you can identify them by searching by word "Needs upgrade" in this file.
Secondly, run the command in PowerShell:
For resuming the upgrade, you’ll need the ID for the database:
Get-SPContentDatabase -Identity <your content database name>
When you have the GUID for the database you can then resume the upgrade:
upgrade-spcontentdatabase -id <database GUID>
Finally, check the upgrade status by running stsadm again:
stsadm.exe -o localupgradestatus > upgradestatus.txt