It could be because the flash is firing its preflash before the shutter opens to get it's exposure, and because of the sheer size of the place you're trying to shoot it's just blasting away. You can try to take it out of TTL and cut the power down, and/or also take it off camera and use your built in as well. I've found that I still get the best exposure by doing it myself! Manual mode, set the built in to 1/16th output, and trigger the off camera at full power has worked like a charm for me. Be happy that in the digital world, you can check your exposures and adjust in a matter of minutes.