In mobile Safari, you can have multiple "windows." If you tapped on a link and it opened a new window, you don't tap back, you have to tap on the window icon in the lower right and swipe left/right to get back to the window you had open previously. Also, if you closed all the windows and you're left with a blank page or your home page, you also will not be able to tap the back button. What happens if you type a new URL into the address bar, does the back button light up?