Karma,

If possible please post a photo. It would allow for a better
chance of offering a solution instead of a lot of guessing. I think you may
simply are seeing mature flower heads that have reach the end of their normal
life cycle.

Kent

I have this problem on my Brazilian cannas. The buds form but blacken and shrivel and fail to open. I think it is associated with caterpillar damage. There are two types of caterpillar as far as cannas are concerned: leaf-rollers and non-leafrollers. Leaf-rollers are easy to deal with, just squash them with your fingers inside the rolled up leaf. The latter is a moth. The caterpillars are very small and only active at night so difficult to spot. They hide between the flower stem and the upper leaves during the day and cause the plant to exude a yellowish goo around them but it's not an effective defence. Apart from the leaf damage you will also see small specks of black excrement (poop) on the stem.

Prevention is better than cure. Spray the plant early with Neem oil, forcing a jet down between the flower stem and the leaves if it is already forming. Sprinkle crushed egg shells and coffee grounds round the base. This is especially good for pot plants. I haven't conquered this problem yet, as if you have these moth caterpillars they are a b****** but there has been much less of a problem this year.