In general, the reasons can be many.
The first is not objectivity. "I think", "I see" - is a personal feeling, without analysis and detailed study of change. According to the rules of experiments, you should always compare two plants, preferably more.
The next point is the type of plant. Some results show immediately: radishes, potatoes, corn and the like. There are plants that need to be monitored for years to record the result. This is due to the life cycle of the plant: tomatoes, conditionally, live half a year, and ficus about 15 years. Keep that in mind
If we talk about the houseplant, you can inadvertently limit growth. Often people forget that a larger pot is needed to grow a plant, because it limits growth. In this case, all the changes could be rooted.
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