Car won’t start on hill

My son drives a 2013 Nissan Pathfinder gifted by his aunt. 2013 was the first year of the unibody "bubble" look. It's got 115k and came with an eye popping stack of receipts. We got it six months ago. 

My driveway is steep and the car is usually parked on the slope. Nose downhill. This week it wouldn't start. Battery is 3 months old. It cranked and cranked but no start.

I shifted it to Neutral and rolled to a flat area.. and it started. After parking on level ground (the street) for a few days it parked fine. Then he parked on the hill again and no start. Rolled it down the hill and it started. 

I am looking for suggestions on why it wouldn't start on the hill. Thx

How full was the gas?  Maybe the slope uncovered the pickup...?

Just one third full this week. The thought had occurred to me. I will tell him to keep it above half full. 

OHSCrifle said:

Just one third full this week. The thought had occurred to me. I will tell him to keep it above half full. 

That seems like a good trouble shooting step, but not necessarily a long term fix. 

Does it stumble but not catch, or just crank with no sign of life?

How much cranking once you get to level ground?

How is the fuel pressure when the no start condition happens?

If no fuel/fuel pressure goes away with the higher fuel level then I'd start looking for why, so it doesn't happen someplace other than the driveway. Some things to consider are bad/broken pump pickup, bad rollover sensor/relay, bad check valve/regulator letting the fuel system bleed down on a slope.

If the fuel level doesn't fix it, or there is fuel/fuel pressure when the no start condition occurs, I'd look at taking a sample of the fuel it happens. I'd be looking for water and would let the sample sit to see if it separates, with thought being maybe there is water in the tank and when level it's evenly distributed across the bottom of the tank on level ground it's below the pickup or mixed in during driving, but on the slope the angle causes the pickup to be in water instead of gas causing a no start condition. 

In reply to No Time :

Thanks for that. Wife and I are out of town and he is using my wife's car right now. I will use my OBD2 scanner when I get home and see if anything shows up. That also might allow real time fuel pressure reading.

I am not 100% sure about the cranking vs stumble sound on the slope. On level ground it started pretty quickly after a slightly longer than usual crank. 

You noted its parked nose down. Try nose up and see if there is a difference at the same fuel level.