Before attempting to import the demo content, please be sure that your server fulfills the minimum Server Requirements.
Also,please follow the instructions in the Import Demo Data post.
Successfully importing the data into WordPress is not something we can guaranty at 100% for all users. There are a lot of variables that come into play, over which we have no control. Most of the time, the main issues are bad shared hosting servers.
Please note that our themes include a 3rd party plugin to offer you to import our demo data. If you any issue importing the demo content using the "One-Click Demo Import" plugin, please check the plugin troubleshooting documentation.
If you have issue importing the demo content using the "One-Click" feature, you still can use the old fashion way described here.
After testing several alternatives to the default the one-click import, we found that the WordPress-Importer v2 plugin works fine even for hosts with low server settings. To use this plugin instead of the default importer plugin, deactivate "WordPress Importer" and install and activate this one instead. For more details, please read the "Import Demo Content Manually" section in this post.