Find out how much memory is allocated to PHP and WordPress. You can find this by accessing the system information from Bulk WP -> System Info. In particular you have to look under PHP Memory Limit and WordPress Memory Limit.
If WordPress Memory Limit is less than PHP Memory Limit then you can increase it by adding the following line in your wp-config.php file and replace 256M with the value that is there in PHP Memory Limit
define( 'WP_MAX_MEMORY_LIMIT', '256M' );
This is the first step. If this doesn’t solve it then you might have to find out how much memory is available in your server. Contact your web hosting company and they should be able to tell you how much memory is available for your server.