This is a common and real issue that should be addressed. I have tried a lot of different approaches, including two speakers (didn’t do much). However, the quality of the center channel speaker seems to make a noticeable difference. I am currently using a single Audience 1+1 speaker as my center and confirm that it makes a noticeable difference. I can at least understand the dialogue most of the time now.
N.B. I use Audyssey for home theater.
You can't hear dialogue because it doesn't matter
I've made several speakers in my life including 2 center channel units. People always ask me if it improves dialogue. My answer has been, no, not really, except in the ability to turn the center up relative to the L and R. What improves dialogue is an excellent frequency response and lack of colorations especially in the mid-bass. Centers are ideally for wide-angle listening.
Here is more on that subject.
This article from NPR says that some directors literally don't' even care if you can understand the dialogue. You just need to get the mood. We've gone from directors who don't care about the plot, to those who don't even care if you can hear the lines.