A pot of pulses that stays firm after hours of simmering is a common and frustrating outcome. The cause is almost always the age of the pulses rather than the cooking.
Storage changes the cell walls
Dried pulses are not inert. Enzymatic and chemical changes continue slowly during storage, and they act on the pectin holding plant cells to one another.
Over months and years those pectins become more resistant to breaking down, so the structure that should soften during cooking increasingly does not.
Heat and time cannot undo the change, which is why extending the cook only produces pulses that are simultaneously split on the outside and hard in the middle.
Hard water and acid make it worse
Calcium and magnesium ions cross-link pectin and strengthen it, so cooking in hard water slows softening independently of the age of the pulse.
Acidic ingredients have a similar effect, which is why tomatoes, tamarind and vinegar are conventionally added after the pulses are already tender.
A mildly alkaline environment works the other way and speeds softening considerably, which is the reason behind the traditional pinch of bicarbonate.
Soaking does less than it is credited with
Soaking hydrates the interior so cooking is more even, and it reduces the total simmering time, both of which are genuine benefits.
What it cannot do is restore pectin that has already changed, so an old pulse soaked overnight still resists.
Soaking also removes some of the oligosaccharides responsible for digestive discomfort, provided the soaking water is discarded rather than used for cooking.
Salt timing is widely misunderstood
The traditional instruction to withhold salt until the end has been examined repeatedly, and salted soaking water in fact speeds softening rather than preventing it.
Sodium ions displace some of the calcium and magnesium in the cell walls, weakening the cross-links that would otherwise hold the structure together.
Salting early also seasons the pulse throughout instead of leaving it bland inside a well-seasoned liquid, which is a separate and worthwhile gain.
Buying and storing for the problem
Pulses carry no reliable indication of age, since packaging dates report packing rather than harvest, so turnover at the shop matters more than the label.
Buying from a supplier that sells quickly, in quantities that will be used within months, avoids most instances of the problem.
Storing them cool, dark and sealed slows the change without stopping it, so a large bargain sack is rarely the economy it appears to be.