This happens because your server stops execution after a certain time. You will need to increase the PHP timeout. Follow the methods below:

Method 1: Edit wp-config.php file: You will need to add the following to wp-config.php:

set_time_limit(300);

Method 2: Edit file .htaccess: Make sure to back up .htaccess before you edit it.

After backing it up add the following to .htaccess:

php_value max_execution_time 300

Method 3: Edit php.ini: Add the following to php.ini:

max_execution_time = 300