When a painting isn't working, check the tonal range in it. Focus on tone or value, rather than the colors in the painting. It may be that the range of tones in the painting is too narrow, or incorrect in terms of aerial perspective .
An easy way to do this is to take a digital photo and then use a photo-editing program to turn it into a grayscale photo using the "remove color" function. If the tonal range is very narrow, add a few highlights and darks.
If you look at the photo above, you'll see how close in tone the yellow, orange, and red colors are, while the green is comparatively dark in tone.
