I bought 4 flamingo willow trees just over a year ago after seeing my in-laws one flourishing in their garden. Initially they looked fine after being planted in the ground but a few months later they had aphids and started to deteriorate, that was last summer.I treated the aphids and this summer they are still struggling. The ends of most branches are dying back almost like it has a disease, I have tried pruning the black bits off but it just keeps coming. I have even tried pruning them right back but it still continues. New growth keeps coming but they don’t seem to be thriving like my in-laws or my parents that I bought one for. Any ideas what’s wrong?