The short answer
The usual causes are adding salt, tomatoes or imli too early, not soaking whole daal, very old stock, or hard water. Cook the daal soft first, then add salt and sour ingredients. A pinch of baking soda can help stubborn chickpeas.
Common causes
- Acid too early: tomatoes, imli or lemon toughen the skins. Add after it is soft.
- No soak: whole daal and pulses need it.
- Old daal: very old stock dries out and may never fully soften.
- Hard water: minerals slow softening; use filtered water.