Cannas are subtropical plants, How long have they been in the container? Is the container sitting on concrete? Is it up on feet? Is it in the sun or shade? What has your weather been like?
Containers can need to be watered as much as three times a day when it is hot! They don't like wet soil over night because that turns the rhizomes to mush. Cannas need sun, but can do very well with some afternoon shade in hot climates. The reflective heat coming up from a concrete patio can be enough to boil the roots off plants so lift the pot off the pati or sidewalk using feet or bricks.