One of the members in a group I belong to, inkjet transfers group, mentioned that a successful work of art typically contained 5 tones. I hadn't heard that but I checked out one of the referenced sites and found quite a bit of good information. I especially liked the idea of taking a digital photo of a painting that isn't working and using a photo-editing program, such a Photoshop, to "turn it into a grayscale photo using the "remove color" function" and seeing if you have an adequate tonal range. I have abstract paintings that aren't working and I suspected it was due to a lack of focal point, but I've seen other abstracts that held together without such a focal point. The tonal range is definitely something to check out and see whether that could help resolve some of them.