I've just finished judging a photographic contest for medical residents (doctors in post graduate training). Not uncommonly I get asked to evaluate photographs and I look at the photographs of photographer friends.
Often I can see what they are getting at, what attracted them to the scene, yet somehow the image is lacking. It isn't a matter of compositional "tricks", lines that cross, things that go to corners, etc.. Rather the problem appears to be that the message is masked by extraneous detail, distracting elements or perhaps most commonly the sin of taking on more good stuff at the price of complicating, not to say muddying of the message.
So, if your images aren't as strong as you'd like, go through your images and see if you have committed some of the faux pas listed above. Have you been guilty of adding the fifth fence post only to accept a tree that spoils the repetitive pattern, did you add that lovely rock on the left only to get a distracting bright sky through the trees in the background, or did you find something interesting but not find a position from which to photograph it that is uncluttered?
Unfortunately sometimes you won't find the ideal view and then you have a choice - shoot the image anyway or walk on. When I'm shooting, if I'm not sure about the strength of an image, I might well shoot it anyway. If I know that there is a fundamental flaw with the image and I can't think of a way to overcome it in the field or in post processing, then it's time to walk.