Hi Dave

In the last part of your posting I think you zeroed in on what I think he meant to say.
Not to discount any adjustments either.

It’s when its warmed up, like when out driving around, it is failing to shift down into the lowest gear possible.
So, When warm it doesn’t have a low or first gear to take off in.
The wording “Not to start” confuses the minds with the starting up of the engine or “not cranking” for a better term.

I know from instances of past automatics, in many others vehicles, that this happens due to either low fluid or dirty fluid, that you stated.

With Really low fluid you can lose the ability to even go into reverse. That’s usually the first symptom for that!
You cannot trust it to can get out of a parking space backwards, especially in the colder mornings, until the fluid expands to make more volume.
It goes away just enough times and then smacks you in the forehead, for not remembering that a few days ago!

The other symptoms of an aged or dirty fluid, makes the shuttle valves stick and not shift positions when slowing down to under 10 mph.
If he listens very carefully there will be a low sounding whine noise when the planetary gear set spins up during engagement and the car will slow down farther.

The outer drum of the planetary gear system needs to be grabbed tight. This makes the highest torque output or low gear for a take off! Here we are most likely.
The shuttles operates different bands, around the few hubs in there, to make gear changes.
It’s a combination of speed or fluid flow that triggers things.

The transmission would bang on the hood if it could!
Like in the movie “Midnight Cowboy, with Dustin Hoffman when he says “I’m walking here!”
The car goes with “I’m needing some service attention here!”

We both believe it’s not happening for fluid reasons first or an adjustment to a clutch drum actuation lever, in the extreme. On most you Loosen and torque back to spec.
On some older Ford transmissions that was an external one, done during servicing.
For for others it’s above the oil pan.
To go all the way up there is Not so easy, but if a worthy oil change is needed, it’s an oil change!

I’m about to change my fluid @27,000 because I’m starting to notice that it “IS” shifting, occasionally, late at various times.
That’s my Hello!
It says, “Are you feeling me now!”
That one is in Too many movies to count.
(:-)

Phil