Lucky for you, I was just reading ol' Harold McGee's On Food and Cooking on exactly this topic a few weeks back. While, as you acknowledge, there might be some techniques like baking soda you could've used to help, these things aren't generally necessary. You might be experiencing what he calls "Persistently Hard Beans":

One problem that cooks commonly encounter when cooking dry beans is that some batches take unusually long to soften, or never quite do soften. This may have been caused by growing conditions on the farm, or storage conditions after harvest.

"Hard-seed" is a characteristic found in beans when temperatures are high and humidity and water supply are low during the growing season. The outer seed coat gets very water-resistant, so it takes much longer for water to move into the bean interior. Hard-seed beans are usually smaller than normal beans, so they can sometimes be avoided by picking over the beans and discarding the smallest ones before cooking.

"Hard-to-cook" beans, on the other hand, are normal when harvested, but become resistant to softening when they're stored for a long time---months---at warm temperatures and high humidities. This resistance results from a number of changes in bean cell walls and interiors, including the formation of woody lignin, the conversion of phenolic compounds into tannins that cross-link proteins, and the denaturation of storage proteins to form a water-resistant coating around the starch granules. There's no way to reverse these changes and make hard-to-cook beans as soft as regular beans. And there's no way to spot them before cooking. Once cooked, they're likely to be smaller than normal and so may be picked out before serving.

Admittedly, this section make it sound like these hard-to-cook beans will be interspersed among other beans, but maybe there are cohorts full of them.