Soaked chickpeas or beans usually fail to plump up because the soaking water is hard, or because the legumes themselves are old stock that has sat on a shelf too long; the fix is stirring a pinch of baking soda into the soaking water and letting them soak a few hours longer.
Why do legumes fail to plump up?
Chickpeas and beans swell by drawing soaking water in through pores in their skin; if the water is high in calcium and magnesium — hard water — those minerals stiffen the cell walls in the skin and slow that water absorption down. That is why legumes soaked in hard-water regions swell more slowly and less fully than those soaked in soft water.
Age matters too — chickpeas and beans that have sat on a shelf a long time lose moisture and toughen up, and that toughened skin resists letting water back in. Legumes from an old package can end up noticeably less plump than fresh ones after the same soaking time.
How do you rescue legumes that did not plump up enough?
Stirring a pinch of baking soda into the soaking water helps water penetrate the skin more easily and speeds up swelling — a trick that works especially well in hard-water areas. Extending the soak from eight hours to twelve or fourteen can also make a real difference, especially with older stock.
Boiling legumes that never fully plumped up can take far longer than usual; adding a pinch of baking soda to the boiling water as well helps shorten that cooking time. A pressure cooker is another practical fix, tenderizing under-plumped legumes in a reasonable amount of time.
What should you do next time?
Making sure the soaking water covers the legumes by at least twice their volume ensures there is enough water available throughout swelling; too little water can leave them stuck mid-swell once it runs out. Using a fresh, recently dated package is also the simplest way to get consistent results.
Keeping the soaking water at room temperature speeds up swelling compared to soaking in the fridge — cold water makes the pores in the skin open more slowly.
Does this apply to every kind of legume?
Chickpeas have a slightly tighter skin than beans, so they generally need a longer soak; thin-skinned legumes like lentils cook in a reasonable time even without soaking at all, so the swelling problem discussed here mainly applies to chickpeas and dried beans. Darker beans like red or black beans, with a different skin structure, can need a somewhat longer soak than white beans.
Opinions differ on soaking in salted water; some cooks believe a little salt in the soaking water softens the skin and helps swelling along, though that effect is far less pronounced than baking soda's and is usually treated as a secondary measure at best.