I am really stumped as well. In all the reviews I've read no one ever complained about this issue, so I am starting to think it is a mismatch or fault with the speedlite. I do have the 430ex II. I am hesitant to think its a camera settings related issue because I can reproduce problem without the camera at all. I just attach the flash to the receiver and hold it in my hand detached from camera while pressing the Test button on the transmitter and watch it fire. I got another set of the same exact triggers from calumet, so now I have two sets that don't work on my flash in low light. But I am thoroughly puzzled by this. Next step is to try a different flash . Perhaps I'll go for a used 580 .. Thanks for your suggestions and for taking the time reply!