I do not use AI often, but when I do it is mostly for maps and the drawing of artefacts. I have a version of CS5. Until recently, I could use the pen tool or the paintbrush to apply the fine or coarse sandstone swatch that would produce dotted shading - the more applications of the tools in the one spot would produce a darker shade as more dots were added, and one could produse a great effect of shadowed and lighted areas.
A few months back I got a new computer; and now when doing the same action using the sandstone swatch there is no addition to the density of dots apart from that put down in the first application. I can no longer produce a combination of many applications.
I have tried adding an extra layer, but that does not work.
Can anyone tell me what I need to do to have mutliple applications added to a single area?
You can apply multiple fills and strokes in the Appearance panel, and you can even rearrange them to change the stacking order. There's a tutorial covering the Appearance panel in Illustrator CS5 at https://www.youtube.com/watch?v=G7Ky-xfwS3w