Every WordPress site must have a robots.txt file.
That file tells to the search engine robots to index or not index some parts of your site.
In a WordPress site we want to index all post but we don't want to index the admin section (wp-admin) or core files (wp-includes).
So How to optimize your robots.txt file in WordPress?
Create the robots.txt file in your root installation folder if you don't have it already.
Fill the robots.txt file with this lines:
This lines will allow to index all your site but will disallow to index the wp-includes and the wp-admin folders.
Some WordPress developers also block the wp-content folder. It's up to you to index that folder.
Here is my robots.txt http://php.quicoto.com/robots.txt
Do you have a robots.txt file?