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I would initially say your mix is too rich, and you probably burned the roots. Not enough water may also have contributed to the failing plants. It is hard to tell if the mulch that you purchased was tainted, but that is also a possibility. I think you have taken the right steps to revive your plants. By the way, a product called Revive might also be useful, but keep in mind it is primarily for gr and secondly, Revive is a bit expensive. I use the liquid form myself. Good luck.