still wont sing

hey guys ive posted before about gettin my bassets started on rabbits, there doin so well at stayen on the track and finding it, but they still just wont open up, they will bark at the rabbit in the roll cage, but even after i let them see the rabbit in the cage and tease them a bit, I'll lay a nice trail that they cant see, but still silent when I let them loose. I tried a sight chase but it didnt go over as planned the rabbits from the feed store DONT RUN. any suggestions will be really helpfull thanks and good luck!