i had a pup that was doing this the same things as you said yours is doing she was bull grey i tried and tried with her and she just kept doing this with a skin on a normal field i thought it was down to being a pup so i left it till she was lamping and when she caught a rabbit she used to kill it then as soon as i whistled she would come back with out the rabbit so i would put her on the slip go pick it up did that about ten times and then she started to retrieve gradually closer and closer then she was bringing them in natural as soon as i would turn light off live to hand as well obvi had a