Thanks for writing. Without being able to see your plant, I can't say why it's not doing better. Has your area been cooler than average? Here in Iowa, our cannas are doing very poorly because haven't had our usual heat for the summer and the cannas just haven't grown at all because of it.
The wetness shouldn't be a problem for cannas --- in fact, they can grow well in wet soil. If you planted them late in the season, the cannas may not have been very good any more (they decrease in vigor if they're not stored properly before planting) and many retailers don't give them optimal storage conditions in the stores.
--Justin, Senior Garden Editor, BHG.com