The most likely reason is that the dog doesn’t understand the verbal request (cue) in the current context, if at all. We find that very often when a dog responds to a cue it probably isn’t responding to the exact word at all – often, it’s responding to the tone of voice and the body language that accompanies that word.
In order of priority for our dogs:
1 The physical communication (body language & gestures)
2 The tone of voice we are using
3 The actual word itself
Now, I’m going to let you into a little dog training secret! As dog trainers we won’t name a behaviour (such as sitting) with a word until the dog completely understands what we want them to do. We’ll teach them the process of sitting, then we’ll teach that sit on different surfaces, then we’ll teach sitting in different locations, then gradually with lots of different distractions round. Then and ONLY THEN will we actual start calling it a ‘sit’.
Teaching it this way means that the dog will very rarely get a sit wrong. Ultimately every time we ask for a sit we are either going to strengthen or weaken that cue.
If we ask for a sit and the dog sits, we increase the likelihood your dog will sit in the future when asked. If we ask for a sit and the dog doesn’t sit, then we lessen that chance. It’s all about setting our dogs up to succeed, then gradually increasing the difficulty.
Train regularly to make it a habit, then practice the habit until it becomes easy!