Gas intake

I have a 2002 GMC Yukon that will not start if the gas is below 1/4 tank. I add some gas, go to a station and fill it up. I can fill it with 20 gallons, it holds 26 gallons so I assume the gauge is correct. What’s going on?

As a guess I would say there is a problem with the inlet to the pump. Perhaps the inlet is sticking up somehow so that the last 1/4 tank of fuel is below that level. The only solution to this problem that I see is to remove the pump and possibly the tank to check things out.

Or the pump is getting weak.

You mention adding gas before going to the station. How much gas are you adding?