I always know when a poem isn't working.
Sometimes I hear it right away and go back and change some words or line breaks or take some fluff out.
Sometimes I don't do anything for a long time and I just hope it will go away, like when my car starts making a funny noise—I want to take to the mechanic, but I'm afraid the repairs will be too extensive and too costly for me to take care of right now.
There are parts of the poem that grate against each other improperly. This isn't the way this was supposed to work: I had a plan, I had diagrams. I know how this is supposed to sound, how it should feel going by the eye and the tongue.
And then, when finally I "get it," when the poem and its purpose become more clear to me over time, I put the poem back together and it's like two intricate cogs whose teeth fit together as perfect as an orthodontic smile.