Do you think they might have taken a hammering from the cold in the winter of 2010/11 then the very long dry spell we have had?
I would feed them up with tomato food and hope they recover. Maybe let them die down in the Autumn and dig them up, speparate them out and replant in the spring.
Dsclaimer: I know nothing but if they have been in the same place for a long time, maybe they need a fresh start somewhere else.