Many plugins and themes need more memory than WordPress or PHP allow by default.
Edit your wp-config.php file
Add this to the top, before the line that says, “Happy Blogging”:
define('WP_MEMORY_LIMIT', '256M');
define( 'WP_MAX_MEMORY_LIMIT', '512' );WordPress memory can be different from the server – you need to set this regardless of server memory settings
http://codex.wordpress.org/Editing_wp-config.php#Increasing_memory_allocated_to_PHP
Edit your PHP.ini file
If you have access to your PHP.ini file, change the line in PHP.ini
memory_limit = 512M ;Edit your .htaccess file
If you don’t have access to PHP.ini try adding this to an .htaccess file:
php_value memory_limit 512M