The "highlighting" or changing of a link to a different color to indicate that you've already read it is a function of your browser. When that stops working, it means one of a few things.
1) Your browser cache is full - clear your cache (just about always the first thing you should try when your browser starts acting oddly).
2) Your browser is set to not save history at all - check your settings.
or
3) You have an add-on that is aggressively clearing your history.