How are you testing it? I was the same with my M41. I wanted to bench test it before I put it in the car. For the OD to work it needs the solenoid engaged and it needs to be turning over. The pressure is generated by an internal pump. I made up a special socket so that I could crank it over with a drill to check that it developed pressure and engaged the overdrive. My electric drill wasn't quite up to the task. It takes a bit of effort. In the end I used a carpenters brace to turn it over. I had a pressure gauge hooked up that indicated it went from a nominal 20psi up to 500 psi when the solenoid was engaged. (this is as per the spec. in the book). I rigged up a bit of wire on the output to strike a piece of sheet metal and got a helper to count output revolutions while I counted revs on the brace. Easy to confirm the nominally 3/4 ratio that way.

Lack of pressure could be due to solenoid not fully actuating, low oil level, blocked strainer or some more serious internal problem. I was particularly concerned with bench testing mine because I tracked down the previous owner of the donor car and the reason it had been put off the road was because the OD stopped working and it was going to be too expensive to fix. Luckily for me that wasn't the case.

If you want I can post photo of special tool to crank it over.