In 1995 I planted a Montana (unknown variety) and it has flowered very well, indeed so well I though I'd plant a second one in a different part of my garden. (this was in 2003)

Both have perished this year, all my other clematis are doing well, (Nelly Moser, President, Dr. Ruppel & Etoile Rose)

Has anyone any suggestions as to the likely cause of death?