Losing Fuel

But after the inlet-valve is open, atmosphere can get into the fuel line, and if a check valve is bad in the pump, then the fuel in the line can siphon back until equilibrium is reached.
Then on the restart, the pump has to either pull the fuel back up, or send it up, depending on where equilibrium was.

But another thing that can happen is this ; EFI cars do not need the VOCs nor a high vapor pressure, to be in the fuel, as the injectors do a pretty good job of prepping the fuel for ignition. So the powers that be, took a lot of the VOCs out so they could sell an inferior gas to the predominantly EFI cars out there. Plus in EFI cars the fuel sits in a closed system, until the car is started.
When our carbyed cars have to use the same fuel, first it sits in a vented tank, with the VOCs evaporating, which reduces the vapor pressure and "thickens" the fuel. Over time, and not much of it, the fuel becomes harder and harder to light off.
So even tho your bowl is half full, and in the old days, would only require a couple of pumpshots to get going; whereas now, you are trying to light the syrup....... if it even goes up the wells.
High cylinder pressure can ease the problem, by virtue of the fact that a good ring seal on a small chamber, "pulls" harder on the wells. And a proper choke setting is invaluable because the low pressure under it during cranking, causes the fuel to spew out of every possible place it can, short of the pump-circuit. Now it's up to the high cylinder pressure to bring the temp up during compression, and then an MS Ignition can come in handy, cuz if the first strike don't light it, maybe one of the following ones will .
If you are stuck with a low cylinder pressure, or less than stellar ring seal,then you can have these kinds of starting problems. I'm not saying this IS OP's problem. I am saying that it could be a contributing factor.

I know, I know; you may not need 11/1 and 180psi, but I gotta tell ya, my engine does and it burns 87E10 for 100% of the time.It is well able to fire up once a week, for a trip around the country, and it has no MS on it. But it does have the finest inductive coil I have ever had; the Big Yellow Square-top Accell SuperCoil. And, and,and it has a double pumper on it,lol.
So, you be the judge.

Here is an experiment for you; pour some fresh clear gas into a small diameter, open glass jar. Level the jar,mark the liquid level, and put it somewhere safe outside, not in the sun..... like under your hood,lol. Come back every two or three days and watch the transformation; the same thing is happening in your float bowl.