It's not unusual to unintentionally teach your dog not respond to certain commands. I see this most often with "come" but it can occur with any verbal cue.
Another reason may be due to the dog having formed a negative association with the command. This is what happened with one of my dogs.
My Beardie developed a negative response to the command "paw" which I used when I wanted to wipe his dirty paws. He is touch sensitive to begin with and I think what happened once is his fur got caught on something just when I said it, causing pain. From that point forward, the association he had with the cue "paw" was a negative one. He became extremely reluctant to ever offer me his paw.

I decided to start at the beginning with the new cue "wipe" and retrained the behaviour. 
Now I can say "wipe" and he willingly gives me his paw. 

If you find your dog is not responding as you would like, consider changing your cue (command) and begin teaching the behaviour again. Choose something your dog has never heard before so the word has no association with either a negative or a positive. You want it to be neutral.

(For more information on teaching come type "recall" into the search bar on my blog page). http://www.sitstaylearn.ca/blog/recall