Oftentimes, when a plant dies, the caretaker may claim they must have a "black thumb" because they
can't keep plants alive. They make sure the plants
get light and water and are insect-free but plants just
don't survive. What might be the problem?

You take care of it too much. Some plants just want to be left alone. Some caretakers worry too much about the plants, they give it too much of the things they don't need and too less of the things they need.

Explanation:

The problem might be the location of the plant, some plants mutations are different from another which means that they grow in a different environment. For example, cactus usually grow in deserted areas, the dried the land the more effective they will grow. Another reason would be that they might actually need some of the insect to grow, just like human needs good bacterias and fungus to live plants might need those as well to survive.

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