HT Guideliness page 45 discusses this in some detail
On page 60 under Serious faults it lists -
3. Failure to deliver to hand.
4. Freezing – refusal to release a bird on delivery for an
unreasonable period of time or until compelled to do so by
5. Hard Mouth – badly damaging a bird, or making it
unfit for human consumption, which in the opinion of the judges
was caused solely by the dog without justification.
On page 61 under minor faults
9. Slight freezing - reluctance to give up a bird.