I have been working with Emgu doing essentially the same thing as you. What I find is that detections returns a 3 dimensional matrix. Being 3D the rows and cols fields are set to -1. To get at the data I had to reshape the matrix to 2D.
The other thing that I found is that most sources say that Cx, Cx, Width, and Height of the bounding boxes are normalized to between 0.0 and 1.0. However, I find that is not the case. I find that the values range from 0.0 to the actual width and height of the image.