If you say that after a feed there is a strong hunt and kill reaction, try putting in a small live mouse, and then try the F/T rat of appropriate size right after when it is still in search and destroy mode. I use a bent up coat hanger (or some people use tongs) to dance the rat around. You could even leave the F/T rat in overnight as suggested above.

Are the rats by any chance a different color? I've heard on the interwebs BPs being picky about color.