safari why can I not use the back button

Hi;

Ever since I upgraded tp Mountain Lion I cannot use the back button to return to an previous website.

It seems I can only use the back button if I am going back within the current web site.

In other words:
I am in website A and go to a website B (home Page) and in website B I open another page, when I hit the back button it will go back to the home page of website B BUT then the back button is gray and no longer allows to to go back to the website A.

Yes, there are tabs that have web pages but they are shortened.

Thanks

Greg