Troubleshoot upgrade errors
Each upgrade process produces an upgrade log file, which displays each error and warning that is found during the upgrade. To successfully complete the upgrade process you will have to fix each error and restart the upgrade process to upgrade the remaining, not yet upgraded, site collections.
Restart/resume the database upgrade
Once you have fixed all upgrade issues, you have to restart the upgrade process. This can be done using the following PowerShell script:
$guid = Get-SPContentDatabase -Identity <dbname>
upgrade-spcontentdatabase -id $guid
However if the database still contains issues, the upgrade will fail again. Review the generated log file to check what the errors are and retrieve more info for troubleshooting.
Verify upgrade status
To verify if all components in the environment are upgraded successfully, run the following command:
stsadm -o localupgradestatus
This command generates a report that contains a summary at the bottom. The part marked bold is the important part and specifies that in all databases there are 16 site collections not upgraded yet, this number should be zero.
 content database(s) encountered.
 content database(s) still need upgrade or cannot be upgraded.
 site collection(s) are contained in the content databases.
 site collection(s) still need upgrade.
 other objects encountered,  of them still need upgrade or cannot be upgraded.
If you send the output of the command to file and search for the text "Needs Upgrade", you will find the site collections that aren’t upgraded yet: