I can say, as an artist myself... nothing kills an artist interest as a worker faster than the lack of a clear deadline of a project.
It might sound like throwing the whole fault at gabe, and i am sure there is more to the story, but it is true, working for a project from a different creator on a 'comission' basis, as gabe was probably only paying him for each drawing, not having him on a payroll, does awaken the understandable fears of the artist of: 'for how long will i have to work on this project? can i start other projects? i need to work in various project at a time, and normally it's fine, but what if all of them ask me stuff for the same time? is it okay if i take a break? I haven't been given new directions for the project for weeks, does it mean i should be looking for another job? how many years will i be working under this conditions?"
I can attest, any artist you work with either will want some form of creative control (at least artistically) to call the project 'his own' too, or will ask for a clear deadline so he can make a plan for himself.
The 'you work for me, but only occasionally and you have to drop everything for my commissions and i ask you for maybe three or four drawing a year, but you must keep your heads up for when I call' doesn't fucking work unless you pay a LOT per piece to justify the uncertainty of the job. ('but it's only one or two drawings per year' yeah, and that's the issue, if the work amount was consistent this would, ironically, be handled better by the artist.)
I don't blame SethXZoe with what has been said if the issue is 'not answering calls is unprofessional' after he has been working in this project for what? seven years? a professional worker doesn't get 'unprofessional' magically out of nowhere, you need to burn him.
In other words... HAVE A CLEAR OUTLINE OF DATES! if the deadlines are set, it doesn't matter if it last a little longer or you need a bit more time, the workers will have a better pace because they'll see a light at the end of the tunnel even when they don't find motivation.