Some beans are not cooked I am cooking beans Vigna mungo  , after soaking beans in water overnight. 

I realized that when i cooked them in morning for 20 minutes in pressure cook, there are very few beans are still as hard as uncooked soaked beans, why these few exceptional beans are not softened ?
Older beans, or beans not stored properly may remain hard. Also, even among fresher beans they won't always soften evenly. Some beans may take 15 hours or longer.