One very important use of the imperfective aspect is expressing the fact that took or did not take place, while the time of its occurence is irrelevant (even if technically it did happen at some specific point in time). Typically called "общефактическое значение" in Russian.
When answering questions or introducing statements, this use of imperfective contrasts to the perfective answer, which may imply that the situation was supposed to take place. Then, you are describing how it went on. In your case the contrast is even more noticeable: "спрашивать/спросить" is, in principle, an action that can hardly end in failure. Thus, the difference in result is negligible (if you engaged in "asking", you are pretty sure to have finished your question). The context, however, will force you choose one or the other aspect.
А: Где Андрей хочет провести отпуск?
Б: Я не спросил его. → Didn't ask when I was supposed to/had a chance etc.
Б: Я не спрашивал его. → Never asked (maybe didn't even think until you mentioned it)
Б: Я не стал его спрашивать. → Didn't ask him (decided not to, even though I considered the possibility).
The concept of "completion" or "result" is not a particularly good explanation of perfective aspect since it only works well for native speakers (who do not need much explanation anyway). The difference is more along the lines of perfective being a "limited" action that is perceived as a point in time without much inner structure, a "simple" action that is characterized by some point in time, with some "change of state" linked to that point.
