This, ladies and gentlemen, is a lesson in tone. As writers, we may have great dialogue, action, what-have-you, but sometimes a scene just doesn't feel right. This could be an issue of tone. A story's tone builds from the first sentence onward. Everything you write effects what comes after it, for good or bad. So, it is important to view a scene as part of the whole.
For this example we'll say you're writing a dramatic scene. If a reader has just read, say, a humorous scene, then has to jump into the dramatic scene, the tone might not yet be properly established for them to feel the weight of the drama. Even if the scene itself is perfectly executed, the reader has to be eased into it. A good way to do this is with a segue, perhaps a bit of description to set the tone. If you describe an ominous or foreboding scene, the reader will be ready to accept that tone. You are mentally preparing the reader for what is about to happen.
From the world of television, here is a good example.
Watch the intro to the show Diff'rent Strokes: http://www.youtube.com/watch?v=m6BxDr6mr3M
Now look at the same video with diff'rent music: http://www.youtube.com/watch?v=dwxLekENDw0
So, this is an example of tone. Exact same video, but the effect on the viewer is completely changed by the tone set by the music. The very first notes of the second video set an ominous, disturbing tone and what the viewer sees is colored by that music, which continues on through the piece. Think about this the next time you sit down to write. Humorous asides are fine to alleviate tense situations but make sure they don't have the effect that adding a funky back beat in the middle of the second video would have. Keep your tone consistent throughout the scene. The humor, if you have it, has to be countered quickly with drama. This keeps the tone consistent and also helps to heighten tension.
Now, you may decide to keep the reader on a rollercoaster ride of funny and tense. This is fine if done intentionally, many writers do it. But be sure it is intentional. If your scene is meant to be sad and dramatic, meant to elicit the single tear in the corner of the eye, you don't want the reader to expect your hero to crack wise while his partner, just ten days from retirement, lies dying in his arms while the man's wife and two small children look on.
Do any of you have good ideas on setting the tone? I'd love to hear about them!