Inquiring minds want to know, so please post the resolution of this problem once you solve it.
I keep coming back to the drop rate valve. You said its open, but I suspect what you mean is the knob turns all the way and then stops. If it was actually closed...i.e., the knob turns but isn't really opening the valve...that could cause this problem. I rememer once't closing the drop rate valve for some reason which now escapes me, then forgetting to re-open it. The next time I tried to lower the 3ph I spent a good five minutes jumping up 'n down on it trying to get it to lower, before I rememered the valve was closed. Color me embarrassed.