Prius V 2012 Wont go into gear until 3 gallons of gas added.

Prius V 2012 Wont go into gear until 3 gallons of gas added.
It started when there were 2 pips and it ran out while driving. Then a couple months later, it wouldn't cold start at half a tank, so we switched to keeping it half full. Now, it is doing it at 3/4 full tank. Im concerned it will continue to doing this after burning only one gallon and we wont be able to put the 3 in to convince the computer its not empty.
Why is this happening? What should I look into for troubleshooting?

Thanks guys

 

Not sure yet, but after lots of research, I believe I've fixed the problem. The symptoms matched low gas but also I kept seeing bad 12V battery. So I had the battery tested and it failed twice. I bought a new one and haven't seen the problem reoccur. I'd like to run the gas down to 1/4 tank now to see if the battery actually fixed it. It could be just coincidentally It was happening after Id been using lights without starting the engine to run down the battery and then the time I was taking to get more gas could have been allowing the battery to recharge just enough to make it start and the gas was merely incidental to the real cause. A case of mistaken correlation vs causality?

 

Nope. Tested it by running gas past half talk. Car failed to start at about 2/3 full. Had to put in 4 gallons and unplug batteries. Problem still exists. I must add, It looks like I dont have electric car mode anymore. The animation that shows where the cars energy is going to and coming from(overhead image of car with motor, generator, battery and wheels) works for a few minutes, then stops. Every time I drive the car. Hybrid light is on. I have several problems all at once apparently. Makes it very difficult to troubleshoot.

 

Well, I have been playing with it all day today. It seems that if I drive it in Brake mode and keep the hybrid battery 4 bar or above, It starts every time. I noticed earlier that when it was failing to start, the hybrid engine was at 2 bars. Then once I finally got it started and rove it for a while, the car was intermittently starting or not starting when the hybrid batt was showing 3 bars. Once I got it to 4 bars, it started every time I tried it, so I drove it in B mode and made sure the battery stayed mostly charged and I have started and stopped the car over a hundred times, no problem. I'll feel better when I get my water pump installed to fix overheating and I get the new AC bc Ive found the hidden fuse in the inverter is blown. Soo many problems...