Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
If you are using a standard always on infrared (night vision) type camera, then what they are using is most likely just a mylar blanket type of reflector to wrap themselves in, it would reflect most of the light and block them from being seen (example: http://amzn.to/2a6ruOh ).
If you had system with a flash, there are some garments that can confuse the auto exposure on a still camera, but that doesn't sound like what you are experiencing.
There are also masks or glasses that some thieves will wear that have a strong infrared light source built into them, which washes out the image that the camera sees.
Your best way around any of these things would be to put in motion lights or flood lights that will do a good job of lighting up the area so that the cameras can switch over to color/daytime mode and better pick up the people .