You cannot use certain constructs in Brushes. The limitations conform to common sense. Grad fills, for example: Illustrator's grad types are quite limited (basically, either linear or circular). So if Illustrator can't, for example, apply a grad fill that bends around a stroke, it logically follows that it can't bend one along the spine path to which a Brush is applied. Same kind of thinking goes for raster images (and raster effects, which are the same thing), clipping paths, live blends, etc.
Some of those constructs can be "dumbed-down" ("Expanded" in Illustrator language) to simpler constructs. You can, for example, Expand a Blend or a Grad Fill to convert it to a stack of ordinary solid-filled paths. The resulting group is then allowable in a Brush. When doing this, though, don't go nuts with it. For example, you almost never need the default 256 steps in a Blend.
This may all sound quite debilitating, but it really isn't. The image below was created with a single ArtBrush, which started out as a circle with a grayscale Radial Grad fill, was Expanded to a few steps, and given a colorization option in the Brush Options dialog:
Thanks for your kind reply. I had already expanded all the vignettes, etc., (64 steps, not 255, thank you) :-) but I think it was the mask that was preventing the brush from being created. Once I did away with that, it finally worked -- in fact, it worked on the very next try after posting my query. :-)
Sometimes you just have to call for help (or whine), and then the problem magically disappears! LOL
I was trying to make a UV Bump Map for my little fish in Lightwave, which is finally rendering. :-)
KidFish0006.jpg 227.8 K