Can't find that ancestor in a certain record? Can't find the parents for a certain ancestor? Write up all the work you have done to find that record or set of parents. Explain the sources you have used, why they were used, and what was located. Pretend that you are writing it for someone who knows nothing about your family and not much about the time period or location in which you are researching.
When you explain something to someone who does not have your familiarity with the details, you are apt to notice gaps. And any of those gaps could be part of your problem.