One of the reason might be:
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:

1) max_execution_time 180
2) memory_limit 128M
3) post_max_size 32M
4) upload_max_filesize 32M

Another reason might be insufficient permissions of php script to create and upload files into your website directory.
You can also check your web server error logs to see what is issue.