DoF Adaptor problem

I recently purchased a DoF Adaptor which was meant to be used for the shooting of my film, this adaptor creates a shallow deph of field that contributes to achieve a film look while working on video, it also allows to attach 35mm lenses to it, which adds several possibilities in terms of lens' choice to obtain different results. The problem is that my Canon HG20 is not able to focus properly on the Ground Glass of the adaptor. I need to zoom into the GG until it covers the entire frame, and whereas the camera focuses it properly when the zoom is back, it is unable to do it when I zoom in. I will keep on trying different options to focus. If it is not possible, I will need to use the University cameras for the shooting.