Are you planting the green shoot or the underground tuber? To propogate cannas, you separate the underground tuber making sure you have an eye. Plant this tuber and it will sprout and grow. If I understand your post correctly, you are planting the green shoot after the bloom dies. If I'm wrong, I apologize for this misread of your post.

I'm planting the new shoot that are 12 inches high and digging it from the main plant and trying to replant it with no success.

When dividing and transplanting cannas during the growing season you need to retain a good portion of the underground rhizome with the divided transplant. If the new transplant has already developed two or more leaves it may be advisable to cut the entire plant back to only a few inches of the soil line. The plant pictured below is about the right size to successfully survive and grow. Sorry about the dumb photographers shadow!

Kent

{{gwi:534154}}

That's my problem. I've been cutting off stalks without the roots. Amazing though how long they do actually live without them.

I meant cutting off stalks with very little root.

My photo above is simply an example of the best case scenario. Under some conditions you may find that you can use a smaller amount of the underground rhizome but I hope you understand the main point of requirements.
Kent