I had this problem with my old receiver, a Sony STRDE-1075. Dialogue was always weak, especially during high impact scenes in action movies. This problem went away with the purchase of my new receiver, so there is one option. (expensive though so try these others first)
Tilt the speaker so that it points at the seating area. This can be accomplised with such high tech devices as a door stop or hockey puck
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Go into the speaker settings in your receiver and turn up the center channel or turn down everything else.
Figure out what center you want and demo it at home on your receiver to see if it fixes the problem. If it does start saving for it.
A well reviewed and very reasonable center channel speaker is available from Bic America, the DV-62CLRS. It has a $269 MSRP but you can get it online for about $100 plus shipping. I have heard it and found it nice for the price and it does seem to be a well regarded speaker especially given the cost. I myself use a PSB 8C and would suggest it to anyone, but it is a little costly and an older model.
Hope these suggestions help, good luck with the problem, let us know how it turns out.