I know why my beans haven't softened - I was adapting a recipe (that calls for tinned beans) to use dried - and there's vinegar in the sauce. I soaked an assortment of beans from different packs overnight, and cooked in the slow cooker on high for 8 hours. That would normally be plenty even in chilli with a fair bit of tomato. As the beans came from different packs that were fine a week ago, and were uniformly too firm, I don't reckon it's that the beans are too old. They were just about edible last night. A further 12 hours on low overnight softened them a little more, but they're still firmer than I'd like. The recipe isn't online but it's beans, onion, garlic, spices, treacle and vinegar, similar to a vegetarian Boston Baked Beans.
The solution for next time is easy - add the vinegar when they're nearly cooked - but I made several portions and would like to make the best of what I've cooked.
Is there anything I can do at this stage to make them soften further? Would neutralising the acid and cooking some more help?