I have many Cannas which flower and send up new shoots but the leaves constantly die off. The temperature range is about 58-72F. Too cold?
Are the leaves dying back after the flowers have faded? If so, that's natural. Just cut the dead/dying stalks to the ground as new growth will replace it.
However, about now is the time that most, if not all, cannas are going to start to die completely back and go into their Winter dormancy.
What growing zone are you in?