It's frustrating, isn't it? You put stylus to tablet ... and nothing happens. The cursor moves with you, but not a thing appears on the screen. Well, there are a lot of reasons why that can happen. I've made a list of things to check, and I thought maybe I should share it. I hope it's useful -- what do you think?

Click here for my tutorial about Preserve Opacity (alpha lock)
Click here for my tutorial about Clipping Groups
Click here for my tutorial about Layer Masks

Edit: There are a couple of other things I've learned since I made this. For one thing, right-clicking with the mouse is the same as using the eyedropper tool to pick up the color under the pointer. That is, uness you're using either the Select or Deselect Brush tool. Then, right-clicking toggles the evil "Exchange Normal / Transparent Color" box! This makes it even easier to accidentally turn that on.

Also, on the Quick bar, above the canvas and to the left of the zoom controls, there's a Selection checkbox. If it's unchecked, your selection will be completely invisible, making it even easier to forget that you have a selection in effect and even more puzzling when you find you can't draw where you want. Fortunately, Sai automatically re-checks that box and your selections becomes visible again as soon as you make any changes to the selection.