Hi JeffA
I just used the GF conical pro for the first time and I'm having the same issue. The coldest I could reach was 5C then it started ramping up. Right now it's sitting at 10C and it won't cool down.
After doing some research it seems like it is a fermenter design problem due to the phisical properties of water.
The maximum water density is at 4C, below that it starts to drop. So what happened is that the top beer got too cold with a lower density than the rest, therefore stopping the convection effect.
This is really disappointing because the equipment isn't cheap. I expected more.