I am a member of Owner group and if I go to any sub site of this site collection I can see tab "site actions" but I can't find it on site collection home page even tho I can go to settings using _layouts/settings.aspx.
It sounds as though you have either modified your master page to hide the site actions, or possibly ribbon, or you provided custom css that is hiding the site actions button.
First, is the ribbon hidden on the site collection home page or just the site actions button?
Second, load your site in Firefox if you can, and using the Firebug plugin, search for the ribbon or site actions container. If neither are found then check out your custom master page. If you can find the proper elements then check to see what css was added that is accidently hiding the element.