Large migration almost always result in some failures. Normally this is not something to worry about, and the first thing you should try after a migration has finished is to "Rerun outstanding failures"
Rerunning failures works the same as resuming a cancelled or paused migration job. It will change the state of all outstanding files and attempt to migration them again.
Inspecting file failures
You can rerun a migration by Clicking the "---" button on the top right of a job and then Clicking "Resume Job/Rerun Failures".
Dealing with Persistent Failures
Movebot will showcase failures and details, including the originating exception from the cloud storage platform in the admin interface. Here you can see why the file has failed to transfer.
If it's still unclear why a file has failed, contact support. We would be happy to help.