My Prius will accept only about 4 gallons of gas after the low fuel indicator begins blinking. I was told in the service department of a local Toyota store that there is a bladder in the fuel tank that might be malfunctioning. I guess the purpose of the bladder is to prevent gasoline from sloshing around by expanding as fuel is used. I don’t really know. The inconvenience of this is that the range of the car is now a little over 200 miles instead of, say, 450 miles. Has anyone suggested a solution short of replacing the fuel tank, a fairly expensive fix?
Trade the car in for another one? That’s another solution. 
But seriously… take it to a shop. That way you’ll take the guess work out of it.
Good luck.
The bladder expand in hot weather so you can put in more gas.It contract in cold weather so less gas is going in the tank.As a rule,don’t rely on the guage but do fill up when it reaches 2 bars.
I try to fill up when I see one bar.
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Bladder issue is a known Gen2 Prius problem, “runs in family”.
Some are affected more and some less.
Try searching on “Prius Chat” forums, they had a lot of people with complaint and some suggested that is you invert the fuel nozzle (rotate 180 degrees, so that handle goes up), it helps to dump 2-3 gallos more into the tank before shut off.
Fortunately (for me), my Gen2 is only around 2 gallons short when fueling (I do not do the “rotate nozzle” trick), getting around 350 miles range.
It could be something other than the tank. As you fill the gas tank the air & gas fumes above the liquid fuel are supposed to be directed to the charcoal canister, which holds on to the fuel for later use, and vents the air. If there’s something blocking that path pressure builds up in the tank during re-fills and this symptom will occur.