Two questions: how far is the illuminator from the camera and are they at the same angle? If it's not very close to it and aimed in the same spot, you won't get the IR light bouncing off the plate back into the camera lens.
You can test this by standing right next to a floodlight and looking at a license plate in darkness outside of the main beam of light. You can clearly see the extra light bouncing off the reflective plate. Move farther away from the floodlight and you won't see the bounce as the light reflects back to the light source. I have an extra 850nm illuminator pointing right where one of my cameras is pointing and it doesn't do much for license plates because it's about 15 feet away from the camera and at a hugely different angle (below and off to the side), even though it is pointing at the exact same spot the camera does. There is a reason external illuminators are placed behind and directly above LPR cameras that do not have internal illuminators. Capturing the light bounce in the camera sensor is what is key, not simply having the illumination pointing at the same spot.
Another issue is that those illuminators probably have a 120 degree spill on them. If there is any way to narrow that beam down they will work much better.
No, I'm certain it's not the angle of reflection. The 940nm IR flashlight can be adjusted to narrow beam or wide beam. In both cases, holding the flashlight parallel to the camera, I had no problem zeroing in on the plate and getting an image. That was my sanity check.
With both the 730nm bulb and the 740nm illuminator parallel to the camera, I saw nothing. I even opened up the iris and gain settings to maximum, and increased the shutter time to the point where I could see the rear of the vehicle, and still saw nothing. And in my previous tests with my sample license plates, I could (with my own eyes) even see the reddish glow of the 730nm bulb reflecting off the plate, and the camera picked up nothing.
My cameras are about 60 to 70 feet away from the center of the roadway, so I'm not pushing the limits of the camera or the illuminators. As I said, I could actually see the reddish reflection of the 730nm bulbs from that distance. Logically, the camera should be picking up something, but it's not.
Tonight I'm going to try one final test, where I'll hold a sample plate a few feet in front of the camera and shine the illuminator on it. If I don't at least see something on the camera, my only conclusion will be that the camera itself isn't responding.
Of course, there is always the possibility that these cheap IR sources that I've purchased direct from China simply aren't generating significant light in the 730nm to 740nm range. I could buy a custom illuminator from Axton for $300 to be completely certain, but I hesitate to spend that much money unless I have some confidence it will really work.