Imagine this, you see your dog "Major" outside digging up your newly planted daisies. You first quietly say "Major, Come here". Major, doesn't respond. You then yell "MAJOR - COME HERE!". Major still doesn't come, so you go over and grab him by the collar and yell "I told you to Come Here!" and give him a smack.
Major, being a smart dog, makes a mental note "Come Here" means "You're in trouble and about to get punished".
Next time when he is outside, and you want him to come inside for dinner you go out and say to him "Come here", he responds by running away. "Dumb dog" you think and go and grab him and drag him inside so he'll eat his food. Then you say sternly "How come you don't ever COME when I call you?!" Major makes another mental note (Hmmm: seems to me my memory of "COME HERE" is still accurate - don't want to respond to that - after all, "COME HERE" means I am in BIG TROUBLE!)
Remember, dogs know ONLY "dog language" when they come to live with us. WE must teach them our language. In the above examples, Major did not learn OUR translation for the word "COME". He learned "COME" meant "angry owner - I'm in trouble now. Better scram out of here!" We want Major to learn that "COME" means "Get your buns to me in the fastest way possible - NO EXCEPTIONS!" Major needs to understand that COME means GOOD stuff - ALWAYS!