Assuming that the electronics are doing what they’re suppose to be doing and the center speaker is electrically undamaged and correctly in phase with your main speakers, the first thought that comes to mind is ‘destructive interference’ of both the direct and reflected sounds arriving at your listening position(s). Some folks have gone a good way to ‘curing this ill’ with acoustic treatments. (See
this interesting discussion and some of the references in it to other information.)
But before spending time and money experimenting with treatments, I would suggest two ‘diagnostic’ intermediary steps. First, at least for a few minutes, listen to the center speaker while standing close to it. Is the dialogue clear as it leaves the speaker? If so, then something is happening in the room after the sound leaves the speaker. Second, carefully relocate the speaker to another location, like on a carpet on the floor, and remove any coffee table or other reflective furniture from between the speaker and the listening position(s) such that the pattern of reflections from the center speaker are radically changed from the way they were originally and the most immediate bounce is likely almost eliminated because of the carpet. I’m not suggesting that you leave the speaker directly on the floor from now on, but just an experiment. The degree to which clarity either improves or becomes worse is indicative of the presence of acoustic reflection interference. Once the center speaker is restored to it’s best placement, it may be time to explore some acoustic changes or modifications to the listening room.
If, no matter where you are or where the speaker is, the dialogue remains unclear, it may be time to think about changes to your system.