Almost every weak tonal drawing fails in the same way, and it's never because the artist couldn't see the form. It's because they never let their tones get far enough apart. The single biggest fix in all of shading is one word: range.
Trap One: Muddy Mid-tones
The most common mistake is timidity. Everything ends up clustered in the safe middle greys — no real white, no real black, just a soft grey fog. A drawing like that reads as flat no matter how carefully the shapes are placed, because your brain needs the bright lights and the deep darks to judge how far a surface turns toward or away from the light. Squeeze the range and you've thrown away the very information that says "this is round." Open it up — commit to a true highlight and a genuine core shadow — and the same marks suddenly bulge into form.
Trap Two: Lost Edges and Over-blending
Tone has two more ways to trip you. The first is lost edges: where two areas of the same value sit side by side — the shadow side of a cheek against a dark background — the boundary should soften or vanish entirely, because that's what really happens to your eye. Beginners panic and draw a hard line there anyway, and the form goes papery. The second is the over-blending trap: smudging everything into a creamy smooth gradient until all the crisp turns are gone. A real form needs some sharp transitions — the edge of a highlight, the start of a cast shadow — to feel solid. Blend everything and you get a soft grey blob; blend nothing and you get a flat cut-out.
The Quiet One: Drawing the Colour, Not the Tone
Last trap: shading something dark just because it's a dark colour, even when light is hitting it. A black jumper in full sun can be lighter in tone than a white wall in shadow. Tone is about light landing on a surface, not the surface's colour. Squint, judge the value you actually see, and match that — not what you "know" the colour to be.