An explanandum is normally complex. Whether it is a single event, a long-term trend, a fact of nature, or whatever, it will have a number of different aspects that a complete explanation must account for.
The rule of completeness is not that a single hypothesis must account for every aspect of the explanandum. The rule is that a complete explanation, which may include several hypotheses of different types, must account for every significant aspect. To evaluate an explanation in this respect, we should formulate each significant aspect as a separate proposition. When we draw up a diagram, we can then see whether each proposition follows from some element of the explanation as a whole. If not, then we should not attribute to the hypothesis more explanatory power than it actually has.