We had this problem and overcame it by spreading 250 gms of Zinc Sulphate, around a ditch, 5 feet away from the base of the tree, 1 ft by 4 inches deep. Do not dig further as you will be damaging the roots too much.
Next for deformed nuts do the same thing with Borax 100 mg added to the above mixture.
These treatments are to be regularly 4 months apart.
Water the area, copiously for 3 days.