Q.indian lemon shrub, lemons fall off
I have a couple “Kagzai Nibu” (lemons with a very thin skin) variety in flower pots. One of them has lost a few leaves (I read your articles regarding loss of leaves). It has flowers blossoming on its branches; however, I notice that once the pollination has taken place and the small fruits begin to form (a few millimeters in diameter or a little larger), they simply fall off the shrub. The other is healthy but does not flower. Hence, I am completely lost and need your help.
Plants are individuals, just as people and animals are, so although they may be the same kind, they don't all behave the same. Losing immature fruit is not all that uncommon. Here's some information on that problem: https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-fruit-drop.htm
One thing that suggest to me is that the pots may not be large enough. This would cause the plants to be unable to grow enough roots to support fruit. Here's more info on growing lemon in containers: https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm
Make sure you are watering properly. Work some bone meal into the soil, and don't use too much high-nitrogen fertilizer (first number.) Make sure the flowers are getting pollinated.