3D Printer
Under-Extrusion
This happens when your print isn't getting enough material through the nozzle. Missing layers, thin layers, random holes, when it looks really bad your extrusion is likely to blame.
The spools are clear? The nozzles are set to the right size? The layer height is good? If you're confident in your settings, but the job still doesn't look right, it's time to check the material itself!
If your material is too cold, it will increase pressure in the nozzle and mess up the layers. If the material is too hot, you might clog or damage the nozzle. Every filament has recommended nozzle speed and nozzle temperature. Usually, when you lower the temperature, you also need to lower the speed. Please ask staff for help if you're not sure!
Pillowing
When the top surface shows these little pockmarks, we call it pillowing. This is always a result of having your shell layer set too thin.
The outer shell of your model should have a "Wall Thickness" that's roughly 6x higher than the layer height you've chosen. If you do that, it'll be nice and thick!
It's also possible that the fanheads are turned off. You should check the cooling settings in Cura and make sure it's enabled!
Stringing
These small strands of plastic happen when the nozzle travels and drips. This is almost always because the temperature is set too high for the material in question. Try lowering the temperature a little bit (and slow down the nozzle speed a bit too), and try it again!
Cura normally puts in a Retraction command, and it should be on by default!
Warping
This happens when the material hardens. As it dries, it tends to shrink. When it shrinks, it lifts the corners and edges away from the build plate.
The best thing to do is to help the build plate stay warm. Use Cura to turn up the heat on the build plate and keep the printer door closed!
If that doesn't help, you should try to make a much thicker bottom layer. The easiest way to do this is to add a Brim or a Raft. The Brim will print a thicker 'throw-away' layer that peels off when the job is complete. The Raft is an overkill option that creates a thick lattice, a scaffold for your entire project to sit upon.