You may have a defective kick down switch. It sounds like everything is working normally except the switch.
The system is wired from the relay through the KD switch to the governor in the normal position. When the switch is engaged it connects the coil(-)and the #6 terminal on the solenoid.
It's possible a defective switch or a bad connection could
disengage the OD and cause the problem you are experiencing.
Fred



1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine