The problem is that it moves just as much backwards as forwards. The ordeal with changing skin colours, which seemed tangential to the main plot at best, took the better part of a year to sort out. During which Agatha had to shut down the castle's mind, as it tried to kill Gil and Tarvek - which in turn was caused by the kill/revive cure required for the tangential colour affliction subplot.
And then they were attacked by Von Pinn, and had to take her out (death ray through countless floor layers - which, as it turned out, took out some vital castle function as well, I don't remember what effect that had), only to revive her and put her into the cat clank body - which again took a lot of time. And then there was the android-like clank.
Meanwhile, nothing seems to be happening outside the castle. Two and a half years ago, the Baron called in the equipment necessary for destroying the castle. Half a year later, they brought in Othar, for whatever purpose. Othar is inside the castle doing something, but we haven't heard of him for ages, except when he met his sister. Last time we heard from the Baron, he wasn't concerned about the castle at all, but was fighting the doctor who tried keeping him in bed.
We're facing the third major fight with Zola. The second fight (October 2010) seemed to have no actual consequences - despite Zola being fed an overdose of Movit 11, and Higgs being stabbed through the chest, they're both present for a re-match in Monday's installment. Zeetha seems injured, but as Gil demonstrated in fights against Vole and Merlot, sustaining major injuries does not have to visibly impede ass-kicking. Even if the current Zola fight is the last, they'll still have to deal with Othar, which will probably take just as long as dealing with her.
Intrigue keeps relying on some shticks that are getting old. Airman Higgs' "survives everything" was hilarious during the first airship flashback, but is getting overused. Zola is always capable of being more powerful than previously believed. Girl Genius is essentially reading like Steven Erikson's Malazan books of the Fallen.
The major plot progression these last three years seems to be Moloch's recent plan to fix the water wheel, aided by the tiny clanks (if they've resolved their non-sequitur dominance struggle). The protagonists still haven't made any significant headway towards connecting the disconnected parts of the castle. Fight scenes are interspersed between various repeated filler elements, including Gil and Tarvek's half-serious rivalry (with Agatha being sent back and forth between them, to share tender scenes with both in turn), comic relief with the castle's mad scientists, and Moloch's inadvertent womanizing. All the while Lucrecia keeps resurfacing, although Agatha seems to be more or less in control now, even without the locket.
I can't really see this story as a monomyth. The five Departure stages can be found in the beginning of the comic, true, but after that everything is chaos. As for the gathering of allies and weapons (which stage is that?), we've had Gil, Zeetha, and Tarvek for ages now, and everyone else is just disposable comic relief. The point is that ever since they entered the castle, the plot stopped moving forward. The heroes are not substantially closer to achieving any major quest goals than they were three years ago.
The characters are occasionally depicted as repairing things, but these repairs don't seem to have any actual effect (apart from disabling traps, but the traps stopped being an issue ages ago, anyway), such as making the castle more helpful, or allowing them to get information about who's where (which would have been very useful for avoiding the constant surprise attacks).