A phrase I often hear when I am working with clients is "I know that my dog knows this, he's just being stubborn".  Hmmmm, not really.  If you've made sure that you have a pocketful of really good things that your dog wants to work for (or a toy, or a favorite game), it doesn't make sense that he would simply refuse to do something, "just because".  If that were the case, his gene pool would have disappeared a long time ago.

Instead of a stubbornness issue, I suspect what you have is a motivation issue.  Your dog simply isn't motivated enough to perform for you.  I talk about this a lot in my private consults and in my classes because contrary to what we've been led to believe, we are not the center of our dogs' universe, especially when distractions abound, so having the right rewards on hand are critical to motivate the dog to want to work for us.

If you do have the right rewards on hand and you're still not getting compliance there may be something else going on.  A common mistake that owners make is to fail to take into consideration that learning is very contextual for dogs.  When teaching new behaviors, it needs to be generalized.  Generalizing behavior means that is taught in a number of situations.

Dogs don't understand concepts like humans do.  If you or I went to a lecture, for example, and the speaker asked us to sit and there were no chairs, we would sit on the floor.  If the seminar was being presented in a room full of chairs, we would sit in a chair and if by chance that same lecture was held in a room where there were only table tops, some of us would sit on the floor and perhaps some of us would sit on the tops of tables.

For dogs it's different.  Behaviors that are learned are specific to the context in which that they are taught.  For example, if a dog learns that in the context of his meals he needs to keep his butt on the ground and not jump around and act like a starved lunatic, that's what he will do.  The food dish then becomes his signal or prompt for sitting.  The same is true for a dog that has learned to sit for a cookie or a treat.  Don't mistake that as "knowing" sit.  For a dog to understand that sit means sit no matter what and no matter where, first he must learn a signal (either a hand signal or a verbal signal) that will elicit that behavior.  As I've said, in the prior example, the dish is the signal for the dog to sit, not very handy unless you feel like carrying one wherever you go.

So let's say that you've taught the dog to assume a sitting position when you say the word sit.  Your job doesn't stop there because unfortunately even if your dog is responding reliably to that cue or signal, unless you've taken your training "on the road", he still may look at you blankly when asked to sit in situations that he isn't familiar with.  If he's learned to sit in your house, you must also teach him to sit outdoors, perhaps on grass and maybe at the dog park.  Typically for a dog to understand that "sit means sit no matter what", you need to teach this behavior in three different settings.

Something else that may be happening if you are getting blank looks when you ask for behavior is that there may be some inconsistencies in your signaling. In one of my classes I asked a woman if I could use her dog to demo sit.  When I asked her if the dog "knew" sit, she assured me she did so I brought the dog out into the center of the room and asked him to sit. No response. I counted to five slowly and asked again.  Still no response.  I handed the leash to the owner and asked her to get her dog to sit.  The owner took the leash and asked the dog to sit and simultaneously pointed her finger towards the ground.  Right then I knew what was wrong.  The dog didn't know to respond to the verbal cue "sit" because the dog had learned to respond to the finger point.  I tested my theory by asking the owner to ask her dog to sit without pointing with her finger.  The dog didn't sit.  Next I had the owner to try and get the dog to sit by simply doing the finger point.  The dog immediately sat. Just as I thought, the dog was responding to the physical prompt and not to the verbal cue. This is what happens often when you overlap signals (a finger point and a verbal cue).  One (most often the verbal cue) of the cues will be blocked or ignored in favor of the more salient cue.  Since dogs are not proficient in English and experts in body language, hand signals make sense to them; which is not to say that you can't teach verbal cues, we all know you can, you just need to make sure they aren't given simultaneously.

So the next time you ask your dog for a behavior and you get no response, do a mental checklist before you blame the dog.  First, do you have something your dog wants to work for?  Second, has the behavior been generalized? And finally, are your signals consistent?  Understanding how dogs learn is critical to getting the results you expect from your dog.