There are some reasons that can make the import process not successful. Please make sure to check on these settings:
-To have the required access on themes files;
-Check the execution time limit.
Change it on your server if not long enough to let all dummy data import. Especially when importing with attachments, the amount of data to load increases significantly;
-Check to have have enough memory limit.
You can increase it yourself by adding this line into your wp-config.php file :
Or ask your hosting provider to do it for you.
If the issue persists, write at our support.