The most common reason I see for this is when the tab history has a redirect in it: going back to the redirect pushes you forward again. If you right-click the Back button, does it show any older history you could go back to beyond the immediately previous page?
The other scenario would be when the earlier history is in a different tab. Or in other words, the page you're viewing was opened in a new tab. Can you find another open tab with the "back" history? If not, if you reopen the last closed tab (Ctrl+Shift+T or right-click on a tab > Reopen Closed Tab), does that have the "back" history?
