I've never worked with a flash before, and have been having some trouble. I have a Nikon D5000 and am using a Sunpack auto 433D. When I take a photo (just to test the exposure) every single on comes out with a horizontal line down the middle, as if to split the photo in half, where the top half is affected by the flash and the bottom is not. Anybody know what I'm doing wrong/can help me out? Thanks so much!