I have a sort of a green thumb for succulents. I've never had any trouble with any of them... except two. My cat usually ignores my plants, but there are two types of succulents that she just can't stay away from. One of them is the jade fingers plant, which breaks very easily, and she loves watching pieces of it fall to the floor. The other is that spiky thing (pardon my not knowing the name of it) from the top picture. I have to plant those under glass so she can't get at them because she loves to just yank them right up out of the soil. I will find the entire plant, roots and all, lying on the floor. That being said... I was wondering if you have a cat? Perhaps an animal is chewing on that one succulent and leaving the others alone, simply because it tastes good or something?