As the others say, there are lots of possibilities, and more info about your camera settings and techniques is needed to help you hone in on the cause(s).
Without knowing more, one of the key clues in your post is the word "wedding." If you were shooting on straight program settings while the bride is in white and the setting is white, there's a pretty good chance that white was dominating in many of your scenes. The light meter and strobe sensor try to average the tonality of what they see, so they are likely to be holding down the exposure to try to turn the whites gray. In that case, everything else is going to be darker, too. I'd bet that in scenes with lots of white, your underexposures ran as high as -2.
When shooting wedding formals we meter our studio lights and shoot on manuals settings, which overcomes that problem. But when we move on to the casual scenes we often switch to on-camera strobes set on TTL with the camera set on one or another form of program. Experiments and experience have taught us to use the +/- control on the camera to adjust for more exposure to compensate, depending on each specific scene and how much white is in it.
Another issue is in large scenes with your principal subjects close to the camera while there's lots of space in the background. The camera is likely to give good exposures on the foregournd subjects while much of the background simply falls beyond the "reach" or power capabilities of your strobe and falls dark. In this case we are forced to either increase the ISO and potentially suffer image degradation while only using the strobe for fill, or to add more light to the larger scene.
All that's speculation, but a pretty good road map of our most common wedding "hot buttons" and issues we plan ahead to overcome. If your circumstances are unrelated, we'll have to know more before we can offer additional interpretation and advice.
Good question, though!
