Both of the previous posts make good points with respect to dialogue clarity. Some additional issues that I think can contribute to the problem are primarily positional in nature. If a CC is not angled properly up (or down) with respect to listening positions, it can be hard to hear as clearly. Although most speakers have reasonable off-axis performance in the horizontal plane, many do not in the vertical plane. That is partly to prevent floor or ceiling bounce from contributing to first reflection issues.
If a CC is located inside a cabinet, or back on a shelf or stand, that can also create problems, as what is around the CC can add distortion to the sound reaching the MLP. If at all possible, the CC needs to be located forward of any obstructions, so that comb filtering will not occur. Of all the satellite channels, proper positioning of the CC is probably most important, simply because it carries so much of a movie's information. And it doesn't take much to obscure dialogue, particularly if accents are involved, or if actors mumble.
Finally, many people put something like a hard-top coffee table directly in the path of the CC, especially in mixed-use rooms. While convenient and attractive, those tables, or other objects, can cause early reflection problems. And as with comb filtering from a shelf or cabinet, early reflections involving the the CC can obscure critical dialogue. EQ won't really solve that type of problem, and may in some cases exacerbate it.
The point about mis-matching an under-powered CC with stronger front speakers is also something that I have experienced. That can also make it hard to hear dialogue. Zorba has long contended that the CC is the most critical speaker for HT and I have never seen (or heard) anything that would contradict that.