Have you noticed that there are times when your dog can't respond to a cue that is normally easy? Have you considered that it may be you?
I've seen this phenomenon multiple times in my training classes. If the sound of the owner's voice is strident, as though the dog has already failed, the dog tries to appease the owner and that appeasement behavior isn't the sit, down, stay, etc... I recognize the dog’s body language, ears against the head, shoulders hunched, licking of the lips, squinting of the eyes, sidling up to the owner. But, sometimes, it doesn't look like this at all. Some dogs jump on you when they’re confused.
Some dogs will use a common “calming signal”, turning the head away and avoiding eye contact. It’s as if the dog is saying, "Hey, you’re making me nervous. You get a hold of yourself and I'll make eye contact when you're calm."
My Golden Retriever is, let's just say, not your typical Golden. When she feels that I am stressed or frustrated she doesn't respond to simple cues, like "sit" or "stay". Instead she slinks over to me, tucks her body close to mine or looks away. If I don’t recognize how I’m affecting her, we get stuck in that appeasement cycle with me slowly eroding her confidence. Then I take a deep breath, smile, relax and change the tone of my voice from "impatient" to "friendly" and she can cooperate normally.
We know that dogs are experts at reading body language, but, they are also masters at noticing when we are emotionally unstable. I love that she helps me recognize when I need to chill out. She reminds me that anxiety and disobedience or two completely different things.