What are you saying?
For almost 15 years I've been teaching people to train their dogs; but I've been watching people train their dogs for over 35 years. I started training at the age of 13; watching people do it right and do it wrong. Seeing frustration level grow between both human and canine I can usually put a finger on the problem. I remember sitting in on several obedience classes in the past; I watched the trainer put the teams through their paces and cringed at a lot of what I saw, miscommunication.
As humans we tend to flap around a lot we move our body parts here and there. If you've ever watched a human and dog team work where it looks like there are no commands being given you will understand how must wasted signs we give. Dogs learn the tiniest of signals; it is truly amazing. Each fraction of a body movement can be used as a cue for a behavior.
Let's take down for example, typically a hand motion over the dogs head height then going down to the ground. Add into that a leaning forward, head tilt and over exaggerated arm motion from the human. The dog becomes accustom to having all of these put together equaling the cue for "down." So when you simply stand offering no other cues except for the hand signal they sit looking at you like "what?" You think they are defying you when in fact you are not giving them the normal cues.
Paying attention to your entire body can be difficult on your own and is usually helped along by having someone else watch. I remember watching a "stay" exercise at another trainers class. One dog and human team was having a really difficult time of it. The dog was brought to the other side of the class; given the stay signal and the owner walked to the other side of the class. This is where it fell apart; the owner kept waving her arms around talking to the dog, you could clearly see the confusion in the dogs face. Being that it was not my class I had to keep my mouth shut but offered a few pointers after class.
One signal means just that, no body lean, head tilt, mouth motion, leg bend or arm wave added to it. Check yourself, when you are speaking to your dog with your body are you being clear?