First thing to try - make sure it's set for internal instead of external fan speed control. If it is set for external, set it to internal and try again. If it still doesn't work, set it to external and ground the FAN pin. If it still doesn't work, I have no idea.
If it doesn't work on internal but works on grounded external, short around the 393 resistor on the backside of the board and try the knob again (on internal). If that works, either permanently short around that resistor or tie a wire from the rightmost pin on the knob to ground.
The other option if it works on external, is rig up an external pot between the 3.3V and GND pins on the header, tap on FAN,and adjust from there.
For the record, this is not a problem with the board. It's a change in internal design spec on a different variant of the power supply than I had to test with. The half-dozen supplies I used all worked, but there exist other variants that, for some reason, use a different voltage standard for fan control. It's not a problem if you have a closed-loop active controller, but that's not what we have.
And since this is the third time I've answered this question today, and at least the eighth time I've answered this question in the last week, I'm gonna start directing people here if they ever mention the word "fan speed" to me in any communications.
Sorry for making your answer the question again - I looked thru the multiple threads where the boards are discussed but didn't come across the previous responses. I've got six of these supplies - but have only tried two of them so far - I'll try some of the others and see if they perform any differently. I can check the version numbers on the labels and see what I have.
If that doesn't work - I can try some of the other suggestions.