Overwatering is really a misnomer. What's really going on is that some types of soil can stay moist for too long, because of the texture of the particles, and they way they lodge tightly together, filling all spaces in the pot. When that happens, there are no tiny air pockets and roots can rot. When a drip saucer is used, the bit of standing water in there can also be lethal to roots. When roots are rotting, they are unable to deliver moisture and nutrients to the leaves, and the effects manifest as yellowing, brown edges, eventual death of the plant.
Learning how to avoid that can greatly increase your success with any potted plants. To read specifically about that, this article is extremely informative.
Bagged potting soil is often not what one would describe as well-drained, especially after significant time has passed. It can settle (in conjunction with some decomposition as well) and suffocate roots as it turns into muddy muck. Meanwhile, the roots have filled the spaces they find suitable, and eventually run out of room to grow.
A well-drained soil... water flows out as fast as you can pour it on, and dries within a few days. The way (stuff to use, amounts of it) to achieve that is the worthy subject of debate, but about the principle, there is a huge confluence of agreement. Ideally, about any (non-cactus) plant would like to have soil that is never dry, never soggy, always well aerated.
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Shaping and pruning
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