I have had problems for years with crown rot, I lose several every summer. Sometimes small clumps that just never took off, I can understand those. That has been what has happened mostly this year. I have lost large, apparently healthy clumps to crown rot though. One thing I've observed, they have tended to be plants that had an exceptionally good blooming year. Almost as if they bloomed themselves out. I've never been able determine the cause, rarely see insects. Mine are grown in the ground though. I have plants that are like your first plant, go dormant after flowering, but come back nicely with rain & cooler weather. Not that any of this is any help to you, but I feel your pain!
