If I had to hazard a guess, Eternity, it'd be that you're looking at your stats for a date range that doesn't include the present day.
It's worth remembering that the date range tool setting is 'sticky' and remains where you set it last - even after you've logged out and back in.
I've taught myself the habit of continually glancing at the date range, every five minutes or so, as I work with advertisers. It's now second nature - sort of like looking in the rear view mirror while driving.