All I can think is that they didn't make the proper root growth last year, despite all the rain, and perhaps got frosted too hard in their pots last winter. Might they have been waterlogged in their pots? They love water, but good drainage, too, being woodlanders. Any lack of root growth would be critical when they finish flowering, put on new growth and call on the roots for nutrients. The sudden request for supplies would be met with silence and the result is the death of the top of the plant. Have they gone completely? Cut back the dead stuff and put them somewhere shady and hope?

tease the root balls out a bit too. so if there is any life left they will also regrow. They may have been overfed too. Excess feed affects water up take by the plant.

Perhaps they were pot bound when you got them - sometimes if the roots are tightly wound round they can't escape without a bit of help. If you decide to try again don't be tempted to buy the big plant in a small pot, and if the roots do fill the pot tease them out gently before replanting.