This issue is usually caused by Memory limit exhaustion. Try increasing the PHP memory limit to double of what you currently have. Let us know if that still does not solve your issue.
If your hosting server allows you can increase the moemory limit from the wp-config file.
Increase PHP Memory to 512MB
define( 'WP_MEMORY_LIMIT', '512M' );
Please note, this has to be put before wp-settings.php inclusion.
