I have experience with pruning nandina in this manner. i say let the nandina recover awhile before pruning again. then reprune the bottom, the nandina should respond with top growth. however, having them in a row like that looks most unnatural. try using them as a screen to hide something of interest so as to encourage a discovery. Meigakure--"may-gah-koor-ay" this is called in japanese.

Matsuo-thank you for your reply. Actually, I was trying to use nandinas for two purposes. The first was to enclose the Japanese garden area and separate/seclude it from other areas. The second purpose is to use it as you indicate, to screen the details of the japanese garden area and to invite entrance and discovery as you approach it from the bottom of the second picture (pathway begins at lower right of second picture).
The nandinas follow the contour of the low rock wall which is not a straight line but they still appear to be pretty much in a line. Should I move the center Nandia farther from the wall for a w shape or perhaps to the other side of the wall and form an elongated curve?