In my experience helping others I think people fail in one (or unfortunately, sometimes two) of two ways:
1. They didn't start off with a healthy animal; they got their pet from a pet store or cheap dealer who sold them a weak, sick, or too-little baby.
2. They didn't set up everything properly from the beginning, at least the most vital things like lighting, water, and food.
But if people start off with a healthy individual and have a cage set up well then usually things will go smoothly.