I agree that it�s probably hoping you�ll prune it, but not that May or June is the time (mine still looks too good at this time of year). See link below for RHS advice:
http://www.rhs.org.uk/advice/profiles0605/wist eria.asp
Personally I do a 2 year pruning cycle which works well for me. One year I do what you�re supposed � one late summer prune and one in the winter (Jan/Feb) and the second year I do only the summer prune but take it back to 2/3 buds. Must be OK because it�s always dripping with flowers in the spring.
If this doesn't work, give it some food. A drop of liquid seeweed should help.