When a painting isn't working and you're struggling to put your finger on the problem, there are various aspects to consider, including everything on the list of elements of composition and elements of art. If tone isn't high on your checklist, it should be. Certainly way higher than blaming the lack of skill of the artist (i.e. yourself) and your tools!
Often what a painting needs is a larger difference between the lightest and darkest tones. It's all too easy to use only mid-tones, which is like a pianist playing only with the middle of the keyboard. And it's all too easy to be afraid of the dark. I don't mean black, necessarily, but the dark browns, blues, purples, greens, and even reds. The deep bass notes. The tones that seem too dark on the palette, that seem frighteningly stark when brushed on and I have to fight the impulse to wipe off in horror.
On a physiological level, our eyes differentiate between colors and tone: the cones in our eyes see color and the rods in our eyes see tone. Cones are concentrated at the center of our field of vision, and are related to visual intensity (the sharpness and degree of scale for what is being seen) and the perception of color. Whereas rods, which give us the tonal quality of an image, are connected to night vision, motion sensitivity and peripheral vision. This relevant to the need for tonal contrast in a painting because tone is picked up in the peripheral vision, so the whole of the painting, not just the small section you are concentrating on, is having an impact on the viewer. Tone makes us look around the painting; a bland, mid-tone painting has nothing at the edge of the eye to draw your attention onwards.
I've learnt over the years that adding a rich dark or a bright highlight is often all a painting wanted. The photos above are an example of this, where I was working on a meter-tall painting in an ongoing series featuring tree trunks. (Click on the photo to see a larger version.) If you look at the left-most trunks in the photo on the left, you'll see they've got some shadow on one edge, but the trunks overall are quite similar in tone. (Squint or half-close your eyes and this becomes more evident.)
I usually only see the lack of contrast when I step away from my easel and look at a canvas from a distance. At arm's length on a big canvas it's easy to overlook as I get distracted by color. The photo was taken when I was half-way through adding a strong dark to one edge of the tree trunks. It's a mixture of burnt umber and perylene green with a touch of red tossed in for good measure; colors I'd used in the background and in the tree trunks. You don't suddenly want to add another color -- unless you then use it elsewhere in the composition.
The center photo shows what the painting looked like when I'd added the dark to all the trunks. I used a small piece of an old credit card (a palette knife does the same job, but often I can't find it when I need it!) to apply the paint. The lack of control with this helps creates an unevenness to the paint application, a result that feels more organic.
The result is rather extreme, somewhat heavy-handed and ugly. But you need to trust yourself, knowing that this is but a stage in the painting's development, not the finishing point. The right-hand photo shows the painting a bit later, when I'd glazed and layered over the tree trunks some more with other colors, reducing the amount of dark visible. The overall effect of the dark to light contrast is now more subtle, but if you compare the right-hand and left-most photos you can see how the overall result is more effect, more visually exciting. So be bold with tonal contrast, not bland! It's an essential element of a painting's composition!