If the import stalls and fails to respond after a few minutes, or it fails with a simple error message like “Import failed,” You are suffering from PHP configuration limits that are set too low to complete the process. You should contact your web host and ask them to increase those limits to a minimum as follows:
You can verify your PHP configuration limits by installing a simple plugin found here. And you can also check your PHP error logs to see the exact error being returned.
Also if possible update your PHP version to 5.5
After doing this you can reset your database using the WordPress Database Reset Plugin and try to re-import the demo data ( WARNING : Reset the database will make you lost all the data of your current wordpress )
If you still have problem your web host may uses process watching software that prevents bulk processing on their web servers.
If you have problem please submit a ticket and we will help you to import the demo.