It looks like you've just been unlucky.
There is a group roll and the player who wins the roll says the dialogue option they chose. Everyone hears that player’s choice and the deal with the consequences of that choice.
This can lead to some interesting conversation dynamics if the leader is in the minority.
One important detail to note is that Light side and Dark side points are not awarded to the group for the choice depicted, but for each player individually according to the choice they made individually.
Source: Interview with developers
This matches what the Developers said when they walked through one of the first flashpoints - The Esseles.
Finally, the 1.1 patch notes allude that the game tries to "normalize" conversation rolls to make sure who "wins" each roll is approximately equal.
Players who spectated conversations no longer unleash an initial massive critical social roll in their next multiplayer conversation.
The above quote would seem to indicate that not getting to speak in one social roll gives a boost in your next social roll, resetting when you finally get a chance to speak.
Numerically, each person rolls a d100 (random number between 1 and 100), with a bonus of 20 for each time you lose. The highest wins.
E.g., d100 the first time you get asked, d100+20 if you failed that, then d100+40 the next ... reseting back to d100 after you win one. (Logically, this makes it impossible to lose six times in a row when traveling with a single person).
You get twice as many social points when you win, as well as the opportunity to decide what actually happens in a quest if hitting an actual decision point.
I haven't worked out what happens with a tie; I've seen them happen and one person wins, but I can't judge why. I'll assume a hidden internal coin-flip, or possibly it picks whoever has lost more at that point.
