Our disease lab indicates in our Pest Management Guide three problems that they normally see on Nandina.  One is a leaf spot, which your plant is not exhibiting.  The other two are root diseases.  Can you examine the roots and see if they are black and brittle or white and firm?  Another problem could be voles and rodents.  Can you insect the base of the plant to see if if an animal has eaten or girdled the stems?