Question about Cooking lentils in crockpot with tomatoes ?

I see so many like lentil soup recipes where you put dry uncooked lentils in a crockpot with canned diced tomatoes and tomato sauce and salt, etc. ie lots of acid.

I tried one of these such recipes three times and each time the lentils were hard. None of the comments mentioned this as an issue. However I have read that lentils can’t be cooked in salt or acid or something like that bc they don’t soften??

I really don’t wanna try one of these recipes again and waste a bunch of food again, but being able to throw everything in and go is very tempting. Are the people writing these recipes just getting lucky that their lentils aren’t hard? Is something wrong with my crockpot? Any tips at all would be great.

For example, tried to make this and they came out hard twice – https://www.thefullhelping.com/slow-cooker-chipotle-lentils/