Just remember not to add salt or anything acid to any pot of beans or lentils until they are soft. Things like salt, tomatoes, vinegar, lemon juice, etc. will slow the cooking process of beans and lentils. This is usually the only reason the beans and lentils don’t all get cooked well enough. Sometimes they will prevent the beans from ever getting past the al dente stage if you add either one too soon.
There is one brand of the mixed beans for soup that I picked up one time that called for tomatoes way too early in the cooking process, but I don’t remember which brand it was. Every person I have ever heard complain about the beans in this kind of soup not all getting cooked added tomatoes or salt before the beans were soft. Once I have told them this hint I learned from my mother many years ago, they have never had a problem again.