We have all the components for our second photo booth ready to go.. except the external flash. We just purchased the impact vsd 160 and connected it via a hot shoe to our canon t2i. When we push test it flashes, but in photo booth mode it doesn't go off. I looked through the camera menu under external flash and it says "The menu cannot be displayed. Incompatible flash or flash's power is turned off." Yet it is on. Any ideas?