Navel Orange Tree

Liz starts with ...
We have a navel orange tree possibly 6-7 years old. Can't remember when we bought it but when we did buy it from the nursery it had curly leaf and we can't completely get rid of that. When it got fruit heaps dropped off which I know happens but I feel too much dropped and now the oranges are big and ripening but lately have started dropping off while still partly green. I picked up 4 today alone. Last year all the fruit dropped off very early and we ended up with 17 oranges. We feed it with MPK Blue and a citrus fertiliser and also this year have sprayed for fruit fly. My husband prunes it every year and it gets terribly thick in the centre but I read today that they shouldn't be pruned apart from in the centre if they are thick. In our old house we had a navel tree for many years and had no problems. When we kept chickens the manure seemed to be good for the tree. Hope someone can help with info as to why the oranges are falling off. We live in W.A.