In the old days, and even still now we would call this print head clogs. Is there a cleaning you can do prior to printing?
If it does it on the black, it's doing it on the flames too. You just cannot see it due to the colors. Try cleaning the print heads and see if that works.
The other thing that it can be is corrupt drivers. Delete the plotter from the system, reboot, then go back and install a fresh copy of the drivers without the plotter being hooked up to the system. If you leave it hooked up, then it will find the old drivers and use those instead. Also i recommend getting the latest drivers for it from the website. Just to make sure you have the latest.
This kind of printing is normally caused by one of those two factors. The last one might be the print head might be bad. But do the cheap thing first before you jump onto something bigger and more expensive.