I would make sure, the bottom of the pot is draining. Black plastic nursery pots can sometimes deform and have water pool in the bottom. I'd also do a quick check of the roots, popping the plant up halfway. If it smells bad, you need an immediate repot.
FWIW, I've had dieback on my amurs this winter. I have taken to giving them no protection in the winter, since they usually became active in Feb. I planted them on a slab last summer. I suspect with all the snow, they stayed too wet for too long. They bounce back quickly though, although big pruning wounds tend not to heal all that much, or even expand. I wouldn't get one with large pruning wounds already on the tree.
