Would that there were a simple answer to that question. Aye, I've pondered it myself occasionally. One could count up the hours of raconteuristic toil and be astonished at how little one is able to reap after soooo much back-breaking plowing and sowing.
That one time I got it? A throwaway one night while I was at work. Just a short answer to Ms. Larson vainly inviting me to play a books edition of Trivial Pursuit, with no attempt to be witty. Prisco said it happened to make him laugh when he read it. There's a couple of pictures I've taken that have struck people's fancy when they seem fairly unremarkable to me. Sometimes you've gotta take your audience's word for it. Not that you should second guess your creative endeavors and think "I've gotta do it the way they like it again!". That is the deadest of ends, and it's easy to get there. But you can't argue when something hits. Your muse doesn't always let you know when she's working, just like my mind didn't tell me I was stressed out and went directly to my abdomen instead. I thought I had an ulcer but it was just my brain coaxing me to calm down, instead of telling me that it was upset in the first place, like a knuckley punch in the upper arm to say "you're swell!"
See? You can't trust your own mind, nor its creative pursuits. And you are not going to just know that this is the one, this is the hit! So just keep working.