The bitmain power supplies are notorious for not starting if the ambient is 10c or under. Also when you unplug the APW3 you need to wait until the fan has stopped spinning on the PSU so that all of the power has disappated from the PSU before it will turn back on. If you do not wait, it will just keep the fan spinning and never output 12v
I'll 2nd this comment. My day 1 experience when I turned on (80) APW3++s at near 0C was that less than a third would boot an S9. At 10C they mostly worked (I found a couple of them that will only power 2 cards).
To this day, if I suspect an APW3++, I take it into my hot aisle for 30 minutes or so and then try it again.
Once they power up, they seem to generate enough internal heat to keep going.