Too many factors to point to any one particular reason Raymo. Could be your lens focal length or contrast, transparency on the night, camera sensor, processing technique etc. I've used an unmodded camera and have been able to capture it. I've read that some actually recommend using an unmodded camera because it is more sensitive to blue which suits this reflection nebula.
I shot these from a bortle 4 sky so your bortle 3 should be better. I do notice that there is no evidence of the nebula in my subs. Did you actually process the stack? You were also two thirds of my exposure so not a direct comparison in integration time but maybe your wider aperture setting makes up for that. All I can suggest with what you have is to stretch the image and increase the contrast considerably just to see if any definition of the nebula is revealed. Good luck!