My 14+ year old mauritius gives me 20 to 40 lychees per year.
The 10 or so year old sweetheart has been a better bloomer (prolly 20% bloom each year), but fruit set and retention is terrible. I've been told that even the slightest drought stress will cause them to abort fruits and / or fail to set. And fruit set happens during the driest part of the year usually.
I'd tear them out, but I like the shade.
Our Mauritius flushed growth. Yawn.
I was told lychees have a lazy root system and to put a sprinkler under the tree and water heavily once a week. This is my first year doing just that. it was also suggested to cut nitrogen as good producers had low nitrogen in the leaves.
A neighbor said fruit trees up North that are poor producers are beat with a chain to an inch of its life, so I did that this year and girdled 30% of the branches after the trunk showed signs of healing, over half of this 15 year old Mauritius tree is flowering.
I did this because only after a hurricane did it produce like it should.