My lab mix puppy will be six months next week. We have been working on private/group training since she was 14 weeks or so. Up until this week she has been extremely easy to train and very food motivated. All of the sudden at basic obedience school graduation a few nights ago she started refusing to go down on command. She is still doing all her other tricks/commands but when I say down, she scurries away from me with her ears back. It doesn't matter what the reward is either. DOes anyone have any experience with this? Advice?