I had the same problem with my BP, struck at the mouse several times and wouldnt eat it and was hissing. I've been told that it was simply stressed. In the end I left the mouse (f/t) in the hide with the snake and a couple of hours later it had gone. That worked for me, it may work for you, but do whatever you're comfortable with. From what I've learned 3 weeks without food is not too much cause for concern, but its a relief when they do have a full belly!
Aubrey.