Gas Gauge problem

I've had my 2005 Prius for a little over a year and this is the first time this problem has occurred.

I just filled up the tank. The amount of gas it took was just about right for the mileage since the last fillup. However, after I started driving the car, the number of dots (5) didn't advance to 10 as they always did in the past. I checked to see that the cap was on the tank properly. In fact, I opened the cap and closed it again. Still, the number of dots remains the same.

The mileage and mpg also didn't reset on the Consumption screen, but I did that manually. At least I'll have that info as a guide to when I need to refuel next.

Does anyone have any idea what might be happening? Is there a way to force the gas gauge to "retest" and display the proper number of dots?

Thanks for any help or guidance.