2003 Tahoe - Fuel pick-up issue

I have a 2003 Tahoe 5.3 Flex. I'm having issues getting fuel to fire. If I get below 3/8 of a tank and let my truck sit for over 30 minutes, it acts like it can't get fuel. I can add two gallons and it fires right up. If I'm driving, I can take it all the way to empty. It just doesn't want to pick up fuel to start. This also happens if I park it on an uphill incline. Any suggestions?

:welcome:
I am sure someone will come along that might be able to help you.