I just watched a Blu-ray movie in 5.1....could hardly understand the words. That's true of tv as well.....sometimes it comes through fine...other times bad. I think they produce it at a low resolution rate.One thing you could try is disconnecting all the speakers except the center, and then placing your head just a couple of feet away from the center channel, on-axis. From there see if the dialogue you had trouble understanding is clear or muddy.
If it is clear in this situation, then the speaker's location, its interactions with the room, other speakers, and your usual seating location are the culprit. Some of that may be addressable to some degree.