Do you also have 24V at the tube's plate pin?
If the grid has a connection to ground (via the pot) and the cathode has the trim pot resistance between it and ground, and you have 24V on the plate, there is not much left to go wrong. Just for laughs, check the voltage between R5 and D2, and then on the other side of R5 (on the DC in side).
As I understand it, if there are no voltage drops across D2 and R5, there is no current flowing at all through the tube. The most likely problem is the grid or cathode. You said you switched tubes and the problem stays in the same channel, eliminating a bad tube. That implies a problem in D2 or R5, most likely D2 since R5 doesn't do much.
I know this is a really stupid question, but are you sure you turned the trim pot enough to start current flowing? I say this because my 12FK6's read full voltage when I swap from 12AU6's, forcing me to do many turns on the trim pot. Just grasping at straws here. You could set the trim pot in the bad channel to the same value as the trim pot in the good channel, measuring resistance from cathode to ground.
One thing you could try is to replace D2 (which you probably don't have a spare) with a simple resistor. It should bias properly with a resistor replacing D2, the trick is to come up with the right value. I'm thinking 24K, for a 12V drop with 0.5ma, but not sure that it's that simple. Maybe someone has a more refined idea if it gets to this.