I agree. Post a DNF. If alot of people post DNFs for the same cache, then I know that if I, as a cacher, can't find it, its probably not me. Well, ok. It probably IS me, but it is less likely. Furthermore, as Starbrand noted, this lets the cache owner that something may be wrong so (s)he can check up on it.
If a cache hasn't been logged in awhile, there is no way to know the reason - is it missing, uninteresting or it just hasn't been solved (in the case of a puzzle).
I assume you know this, but you might also try to widen your search perimeter. I have found caches up to 80 feet from where my GPS was telling me to go. Don't worry - this is not a common occurrence. 20 feet, however, is not unusual, depending on the circumstances.
I have had people email me for hints on mine. I try to help without helping (if ya know what I mean). I think cache owners do want people to find it (otherwise why hide one?).
Good luck.