I'm new to owning cacti & succulents so I'm not at all sure what I'm doing wrong. I've repotted two different kinds of succulents and even though I used cactus soil and appropriate pots for their size, both times they looked fine for a few days then died so fast there was nothing I could do to help them. I mean they completely changed color and collapsed within two or less days. How do I keep my other cacti from suffering the same fate????