This year no yield from my mango tree

Dear experts, I have seen in the leaf with some small black colour balls on top of the leaf. Last one month it is creating and showing like this....why it is created suddenly? Do we have any idea to avoid in future. I am not sure why it is happen suddenly and please suggest what can I do further for resolve this? Last year, we harvested a good amount of mango but this is zero yield. Why it is happening an alternate year. Expecting your feedback n suggestions