Ailing hibiscus

I have a well established hibiscus oiseau bleu but for the past two or three years it's leaves have yellowed as the summer has progressed.  I have used seaweed feed with a bit of success (new leaves forming nearer the base come out in with a healthy green) and it flowers as well as usual.  I wondered if it is getting too much competition from my neighbours nearby large shrubs.  My soil is slightly alkaline clay, which I have improved over the years.  I mulch the shrub with leaf mould each autumn and with my own compost in spring.  I don't want to get rid of my hibiscus but it doesn't look very attractive.  I also can't decide wether it is getting too much or too little water.  It's south facing 

I,had hibiscus that suddenly got old. Looking at it more closely it really got top heavy without me noticing it. It started to lean, got loose at tne root, yellowing foliage and loads of bare branches. It was about 12' high, full of red/white flowers but deteriorated over 2 years. I found it easy to remove. Now, you asked about competition from other shrubs and this is what I think caused problems for my hibiscus. It likes dry conditions but mine was very dry..too dry. I have another in a better position in the garden and this is fine. I think it wise to prune hibiscus every year to keep it compact, etc. I think the plant is inclined to be top heavy.. so. I would try pruning it fairly hard, even now.