One of the common reasons for a migration not completing is the WordPress site running out of memory. The solution is to increase the memory that is available to PHP and reduce the migration step size through the wp-config.php file.
An SFTP or FTP client will be needed. FileZilla and Cyberduck are free options that are available for multiple platforms. Using a Mac? Try Transmit.
Login to your WordPress site using your preferred SFTP / FTP client and browse to the WordPress root.
Then open your wp-config.php file for editing. Copy the following constants:
define( 'WP_MEMORY_LIMIT', '128M' ); define( EE_MIGRATION_STEP_SIZE, 10 );
Then paste then towards the middle of your wp-config.php and save changes. From here, return to your WordPress dashboard (WP-admin) and try the migration again.