Whenever I cook black lentils there is always a hard crunchy piece in every bite while other pieces become soft as they should, I start cooking by boiling for about 30 minutes in unsalted water until almost all the lentils are soft. I tried changing my soaking time and boiling time before I add salt but it didn't help. Could it be that the manufacturer is mixing old lentils with newer ones or is there something else.
When cooking any sort of legumes 1) Sort them (if you don't trust it to be pre-sorted) 2) Soak them (make cooking easier, and also more uniform, since they already absorbed moisture) 3) Now you cook them. I don't simmer in the slow cooker, I usually pressure cook them