We have four camellias and they have never had any feeds or ericaeous compost I'm sorry to confess. They are situated north, south and east but all have at least part shade. We have a deep red, a pink, a white and a red-pink. They all flower well inspite of this neglect. No idea what any of them are as were snapped off the "mother shrubs" by my father maybe thirty years ago.
However, I was told once that the flowers for one year are formed the previous one and so if they are allowed to dry out one summer you get buds and flowers dropping the following spring.
Is it possible that yours is drying out in the summer?
There is also a type of blight - I think that I read about this at Wisley. It's fairly new to the UK and specific to camellia. I think a Google would have the answer to that one.
Good luck.