I'm not an expert, but I do remember reading a thread on permies that talked about encouraging more fruiting (ie more fruit and a longer harvest season) of [annual] plants by picking early crops. This was justified by the fact that a plant's genetic mission is to create one more generation, so by tricking it into thinking that it hasn't yet sufficiently done so makes it produce more fruit in order to ensure that it has completed this task.
I would imagine this would maybe be slightly more true of annuals, which also succumb to old age, and just wear out.
Plants that suffer from a nutritional deficiency might show other signs, such as stunted growth or irregular leaves (shape, colour)...but again I'm not all that experienced, I'm just trying to put some theoretical concepts together in my head.
It might also have something to do with the life-cycle of plants in the environment eh? Perhaps less pollinators are available, less sun? I'd imagine its a pretty complex answer with contributions from many different facets.
At any rate, good luck Lan Wi.
EDIT:
I was looking for the post about encouraging more production from plants and found this simple guide on plant deficiencies, perhaps this would be helpful to you?
https://permies.com/t/46916/plants/simple-plant-deficiency-guide
