Hibiscus syriacus looking sad

This year my Hibiscus syriacus is not looking at all wonderful. It is an unknown variety 'double white' about 4 meters in height and was here when we purchased the property four years ago.  Many of the leaves are quite yellow and have been since April. The flowers are smaller than usual and they are a bit sparse and there are quite a few dead unformed buds which duly fall off.   It has never been pruned because I thought it didn't need it.   I have given it water and two doses of iron since April.   My neighbours' single blue looks really well with dark glossy leaves and prolific blossom.   I have an number of large mature H.rosa-sinensis and they all look fantastic.  Any ideas what I am doing wrong?
 
 
Pamela 
(Costa Blanca. Today 30° Sunny, pleasant sea breezes). 
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