My guess is that between the low amount of dichromate in the tissue and the density of your highlights, the tissue just is not receiving enough exposure in your highlights to harden any of the gelatin -- thus no image. I would just try a 8% solution of sensitizer and see what happens -- it will lower the over-all contrast and hopefully keep your highlights without flattening the other values.

It is difficult to compare results with different tissues, etc. My home-made tissue has a much lower contrast 'grade' than the B&S tissue, as I use a lot less pigment. My negatives are also have a very high DR...never actually measured it, but it they are probably well over 2.2. For an 8x10 (9x11 tissue) I am spirit sensitizing with 5ml of 8% Ammonium dichromate (diluted with 15ml of acetone -- so that equates to 20 ml of a 2% solution).

Good luck!