We'd recommend that you increase your memory allocated to PHP if you haven't done so already. To increase it, please edit your wp-config.php file and insert these lines:
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
You can add it before the line
/*That's all, stop editing! Happy Blogging. */
And also make sure that you have the following on your server configuration:
If you have troubles increasing the PHP memory limit, please contact your hosting provider as they can increase it for you.
I hope this helps.