Very stupid problem and question!

After taking my dart to the track and manually going through the gears a few times I have noticed my 727 is shifting through all gears by the time I get up to 20 mph and it is not downshifting when I floor it. I know this is an amateur question but what in the world would cause this and how easy is it to fix?

The car had a small block with a 727 by the way.