After 4 years against a north facing fence, this thing is now 6 feet
high, but shows no signs of flowering. Although the plant looks
generally vigorous, the leaves don't look too healthy.

http://i55.tinypic.com/141jl0.jpg

The soil is a little on the stony side and it doesn't get a lot of
direct sunlight, but the mopheads grow well in the same location.
Any advice appreciated.