D8 has own power input. Make sure you get power at all (ATX e.g. needs some tricks to get enabled) and polarity is ok and you get a voltage at the other side of the fuse. Just to rule out hardware issues. In host you can show temperatures with output then you see if output should be enabled when measuring.
Also try without heaters connected, just measuring voltage. If firmware turns on dry run mode, it will not heat anything. See firmware log while testing such things.
