I created a new Pattern Brush to test a new symbol I designed, and then decided to create a new Scatter Brush instead. But when I click the New Brush button in the Brushes palette, now the dialog box has 2 brushes greyed out (the Scatter Brush and Art Brush). Previously all of them were available. What would cause that to happen? Thanks!
Scatter brush' and Art brush' have to be made out of created "artwork" (some object(s) on your artboard).
When you go to New Brush, if making a Scatter or Art brush, have the objects you want to turn into a brush selected
You can also just Select the artwork (to turn into a brush) and just Drag it into your brushes and it will pop up asking you what kind of brush you are making. (Scatter, Art or Pattern)