Comment 1 for bug 94571

Are you electing to reformat any of the partitions during the install? If a partition is scheduled for reformat or deletion, the migration program will not import data from it.

If this is not the case for the partitions you're importing from, please run the installer again using the --debug option and attach /var/log/syslog and /var/log/installer/debug. Detailed instructions can be found here: https://wiki.ubuntu.com/DebuggingUbiquity/AttachingLogs