Wedding reception photos are notoriously tricky. Yoga has covered the main reasons. The other is that the built in flash on many cameras simply isn't powerful enough to fill the room, so you'll get shots with the first one or two rows of people really bright in the shot, and an almost pitch black background.
If you have a very steady hand, you could try using manual settings with a slower exposure to capture the ambient lighting of the room, with the flash firing. Too slow though and you might get some weird trails around some of the particularly fancy dancers, but it might be worth trying.
A more powerful external flash might be the best solution. And hold the camera above everybody's heads and shoot back down at the dancefloor, you may get better usage of the flash fire.
Also, the darker the room (and your eyes will adapt and deceive you), the harder the auto-focus will find it to lock onto an object.