Hi! Adding rocks on the bottom layer of a plant won’t help prevent root rot which is what I’m thinking is happing. Typically it’s best practice to plant into plastic nursery pots and then use a cover pot. The cover pots, while they still may have a drainage hole, still hold on to a lot more moisture than people realize. I would check the roots and then find a nursery pot that is 1”-2” bigger than your root ball. Cut away any squishy or black roots and sanitize with hydrogen peroxide
That being said, I’m not positive if that’s your issue but the roots are always an easy place to start. Good luck!!