What do we do when a dog doesn’t respond to a cue, or won’t do a behavior we are asking?
First and foremost, we want to make sure that there aren’t any physical reasons that is making the behavior uncomfortable, or perhaps even impossible. Sitting and/or lying down might be uncomfortable for an arthritic dog (or if the surface is wet/cold/hard/etc.), bathroom accidents might be hard to avoid if the dog has an upset stomach or urinary tract infection, and aging dogs might not be able to see or hear cues as they used to.
Before assuming something is a training problem, be sure to assess whether what you are asking is still comfortable or possible for that individual dog. Dogs can be good at hiding physical discomfort, so it might take some careful observation, but your dog will thank you for it!