I have to agree with BlueDamsel.
Sometimes, I look for a cache twice before recording the DNF, simply to make sure I have not just over looked it. If I have looked for the cache with much effort (either the 1st or 2nd time) and don't find it, I record it. The cache owner needs to know. I have found that many of the owners automatically email with a hint or words of encouragement. Since, I am new to GC that is always nice.