"Remember, dogs are first and foremost a physical species. Verbal communication comes second." I say this to my students very often. I probably say it to myself just as often. I'm constantly impressed by our dog's ability to read human body language, despite the fact that we are two totally separate species (and in my experience dogs are MUCH better at reading our body language than we are at reading theirs!) Therefore, whenever I'm training a dog I'm doing my best to be cognizant of what I'm saying with my body so that I can set us both up to succeed.
Case in point: just the other day I was working with a dog on her recall cue. We were practicing an automatic sit at the end of the recall, which is a great skill to teach a dog that performs "drive by" recalls where they grab their treat and keep running past you. This dog was doing great, coming every single time I called her from about 50 feet away and immediately going into a sit when she arrived in front of me. What a smarty! After about 10 successful repetitions I called her to come again, and she came flying, but this time she didn't go into the sit. I stood there, trying to figure out what had changed to cause her sit to disappear. I set her up to try again, hoping to troubleshoot my training and figure out what had changed. This time, as she came toward me, I realized that I brought my right hand up to my chest slightly, which I hadn't done before. And this time she sat. The entire time I had been working with her, I had been unconsciously cuing her to sit and didn't even realize it! A few more repetitions later and I had faded that hand cue so that she understood she needed to sit even if I didn't cue her to do so.
So the moral of that story is: always be aware of what you are doing with your body while training your dogs. Dogs learn behaviors in contexts and pictures, and when something changes they almost always notice. This is why we have to work on generalizing a behavior to so many contexts, with us standing, sitting on a chair, sitting on the ground, our backs turned etc. so that our dogs understand it's the verbal cue they're listening for, not any physical signs that may overshadow the verbal. If your training has suddenly broken down, ask yourself if something in your body language changed, and make sure to adjust as needed.