The obvious conclusion is you don't have the right model of Prius (see thread comparing 2008 and 2010 models.) The 1.8L, ZVW30 Prius has two major technical advances that directly address low mileage:
1) exhaust heated coolant - for those living in colder climates, this accelerates warm-up to fuel efficient modes. It also reduces the need to run the engine, especially in high wind conditions, to preserve coolant temperature.2) recirculated, exhaust gas injection - this avoids having to run a rich mixture at high power settings to preserve the catalytic converter.
Just go down to your local Toyota dealer and see if you can get a trade-in on your current 1.5L, NHW20 or NHW11 Prius. The production levels are coming back and new, 1.8L Prius are starting to arrive. Meanwhile, the shortage has still inflated the price of your older Prius. Instead of trade-in, you might try Ebay/Craigslist to see if you can get a better deal than trade-in (often the case.)
Bob Wilson