Problem with my A-904-A Torqueflight Transmission

You did NOT say that you had a Manual Shift Kit installed. In which case,
Part-1; With a standard ValveBody;
the Governor is in charge of when the upshifts occur, no matter in what position the gear selector position is in. In other words ; You put it in manual low, and rev it up, but if the driveshaft speed never comes up, then you will not get any upshifts.
Therefore, for now, the problem seems to lie in the lack of governor pressure.
This could be due to;
A lack of driveshaft rpm, or
a failed governor or,
a failed governor circuit, or just plain old lack of fluid or fluid-pressure.
So then, my first go-to, is a pressure test.
>This is way too easy, in your case. Just put the trans in reverse. In reverse, full pump pressure is sent to the L/R band and the Hi-drum. At 700rpm, it should clunk into gear, and by 2000 rpm (stall) it should be locked up and spinning the tires furiously. This proves the following parts are working;
the pump,
the L/R band,
the hi-drum,
the VB, and
The Manual-valve
In reverse the governor is locked out.
So if reverse is working, you just proved everything but the KD-band, is functioning.

>But if reverse will not work, then, more than likely, the pump is broken. To prove it you only have two choices, a pressure test or a physical inspection.
But before you drop the trans, if reverse will not work, make sure the manual valve is connected to the external lever, AND also synchronized to the Manual Valve. If the trans goes into park and locks the driveshaft, AND comes out of Park just the same, then they are connected. Now prove that manual-Low on the shifter is synchronized to manual low in the trans . This is usually done by putting the trans in Neutral, and putting the gear selector also in Neutral, then test fitting for a slip-fit of the shifter linkage into the external shift lever, and adjusting the mechanism as may be required. If these are NOT synchronized, then the manual valve could end up between gears and hemorrhaging fluid back into the pan, instead of that fluid going to it's destination.
And finally, after the above has not solved the problem, then, the L/R servo could have malfunctioned; Pop the pan and VB and do an AIR-pressure test to see if the servo is working and the struts are correctly installed.

BUT
Part-2; With a Manual Valve Body, then the Governor should have been bypassed; and the driveshaft rpm is a Non-issue.
In this case, lack of pressure in Reverse, has to be either;
a lack of fluid, or
the Manual-Valve is Not synchronized, or
a broken pump.
But again;
>if you do have reverse pressure, AND BOTH, no Reverse/no Low; the common element is the L/R band. Pop the pan and VB and do an AIR-pressure test to see if the servo is working and the struts are correctly installed.
Ok so there ya go, see what comes up;
Happy HotRodding