When we started develop our land 7 years ago we got 4 mango plants from neighbours. Didn't understand yet then that they need to be grafted. They were from seeds. Growing very well, nice big trees but no fruits. I have now cut them back and grafted them all from bought trees. Will hopefully get fruit on them next season.
A couple of the grafted trees we bought don't fruit either. A neighbour explained that some cultivars of mango don't like it here. I'm near the coast with no real cold winter and that might be the reason. This winter been way too warm and only a few mangos produce fruit.
Potassium is needed to set fruit. I tried spraying one year without any result. I have bee feeding them lots of mulch from legumes so maybe too much nitrogen compared to potassium in the soil.
Grafting is best done during summer when trees grow fast. But not that easy, takes some trail and error to get it to work. Important to find the right scion. Best if you can find some older shoots with buds ready to grow. Wrap the whole scion with tape to avoid it to dry out. I found this tape to work for me:
https://www.lazada.co.th/products/pvc-fruit-tree-grafting-tape-secateurs-engraft-branch-gardening-tool-2cm100m-i2669291664-s21729316545.html?