It's a common thing when you have a poor setup or calibration for the center channel (Dialogue) to not sound right on a 5.1+ mix. This would be a problem in dolby digital, DTS ES, DTS HS MA, etc. And yes, it varies from title to tile. This has nothing to do with "Blu Ray" but more of playing surround sound fields based on your configuration. 2.0 tracks will always sound better in the case of "I can't hear dialogue" situations. In general, room configuration also plays a lot into it. Sound reflections, bass absorption, etc.
Surround sound is a complex beast, and your setup is less than ideal. That's not meant to be an insult, it is more speaking to that you have a bunch of speakers at a bunch of different heights and distances which are not conducive to proper surround sound. You also have reflective surfaces directly next to where you are sitting (Against 2 walls). Not to mention you have 3 channels of sound coming right next to your listening position which are not the main dialogue channels. This would flood your ears with special effects, music, etc. A 2.0 track would be more audible because your "Left Front" is right next to your head and thats where dialogue would come out. Your setup is the problem you need to overcome if you want to hear a 5.1 mix. You are in a horrible listening position based on where your speakers are.