By default, each time you start a stroke you create a sort of 'sub-layer' in which the stroke will not build on itself. Next stroke, the new 'substroke' can build on the previous stroke. Which is to say, you can't just scribble back and forth on the same spot and gradually darken the line, but instead you must lift and then lower the pen again to darken it.

Is there a way to allow it to 'build up' within a single stroke, similar to shading with a real pencil? I could have sworn I saw an option to choose between these two methods, but now I can't seem to find it anywhere!

I am using Krita x64 v2.8.3 on Windows 7 x64, and my tablet is a Huion H610 Pro. Any help would be greatly appreciated!!!

Edit a couple of minutes later:

Found it. Many apologies for a question I answered myself. Choose between 'Build up' and 'Wash' in the brush properties window, 'Painting Mode' tab. I can't believe I failed to look there...

I'm not sure I understand what you mean. Maybe try decreasing flow and opacity till you get what you want?

ramskulls: I was referring to the tablet input option, 'Build up' vs 'Wash'. It can be selected by going to the brush properties window and selecting the 'Painting Mode' tab from that brush's properties column.

n_n glad you solved it! have fun w krita!