Advice please...

This is a small portion of our large mulberry tree which is probably about 8 years old and 20 feet high. For the last few years it has had this wonderful promise of a crop but it reaches this stage and the fruit drops off before ripening ...much to our disappointment. Dr D remembers childhood holidays at Mulberry Grove on Great Barrier Island and savouring the delicious fruit.
The tree is mulched and fertilised and even verbally coaxed...any experts out there as this year's crop begins to prematurely drop ? ; ((