Posted: 03/03/2014 at 10:56
I'm not a rhodo grower, but if other established shrubs don't flower for me, I give them a watering with soluble potash.
Also, this article has some interesting information - it says that it can take up to 5 years for the Yakushimanum varieties to establish and flower http://www.rhs.org.uk/getattachment/12daaa8e-1ce5-4c40-bae3-00219bcaa465/Rhododendron-yakushimanum-Bulletin.aspx
It also advises not fertilising for the first two years - perhaps you're treating it too kindly - what is it that Gardeners' Question Time Panel often advise? - "a period of benign neglect".
Hope that's helpful