my runs don't get saved to my iPod

When I looked for a recent workout or workout history it says "No saved workouts" how come my workouts aren't being saved? At one time the ipod saved all of my workouts. I ran a marathon in November with the nike + device and it said I ran 28 miles, when I connected the ipod nano to my mac book pro, it didn't register the mileage (got erased) so I have no record of the marathon run. Longest run ever, too. Really bummed out about that. Very frustrating. Thanks in advance for your help.

Chance