You have receive really excellent suggestions from other members already, nonetheless here's my take on your issue.
1. Dialog intelligibility really depends much on center speaker setup. Would you care to post a picture or two on your configuration? That would help the most!
2. Meantime, placement of the center speaker should be made in a way that it is facing seated ear height. Speakers always deliver not only their highest SPL (Sound Pressure Level), but also their widest frequency range when on-axis with our ears.
3. Another thing to consider is early reflections from nearby objects, like when placed on a cabinet top or on a shelf it may need to be nudged off the edge by an inch or so to lessen such phenomenon.
4. Also make sure there are no objects between center speaker and your ears blocking sound, like a coffee table when center speaker is placed low under the TV, almost at floor level. If you can see it, it's OK.
5. And finally if nothing works, you can always turn on subtiltles for a movie!! Guaranteed to work,...or your money back! LOL
Awaiting your pictures! Take care!