A lot of 2.0 track films default to LEFT/RIGHT only when played on a Blu Ray Player, especially if your audio on the player's master settings is set to UNPROCESSED.
I find you often have to change your player's downmix to either PCM or DOLBY SURROUND depending on how they are labelled (they both give the same result) in order to get the 2 tracks encoded to L-C-R-Mono SURROUND.
Even if you are using a multi-downmix processor between the player and the speakers, it does not let you choose Pro-Logic on some films unless you change your downmix on the Blu Ray player settings.
I had a copy of MASTERS OF THE UNIVERSE which was like that - unless you choose PCM as a downmix on your player you only get L and R channels outputting, and even my processor would not change to Pro-Logic, it was just stuck on 2-Channel. Other times then, it may default to 2 channel but you CAN change on the processor, as CLASH OF THE TITANS '81, another 2.0 stereo film did, but with that one I was able to leave the player's downmix on UNPROCESSED.
I could be wrong, but that's how I interpret your problem...
However, the CENTRE channel as you probably know was created by placing sounds equally on BOTH tracks, so you still should get a 'wide' centre effect - so if the dialogue is poor, then it may also be a bad transfer.