some have issues with self-incompatibility, and may virtually never set fruits, i think because the flower opens up, and receives its own pollen immediately, which is incompatible, and also making the situation worse, it blocks the reception of any viable pollen from another donor plant...the solution i've found is to grow several seedlings instead of just one seedling, or make sure the mother tree you got your seedling from is known to set lots of fruits in isolation, from a young age. And the final solution I've found, is to topwork a non-produtive tree, with scions from one that is known to be productive.
most of the trees that have these issues seem to be from the original stock that was imported over 30yrs ago, the recently imported stuff from 2005 to now, seems to be of superior quality, in terms of productivity, and flavor.