Over the years I have seen many dogs become stagnant or experience extreme regression because the handler dragged a session out for 2 hours when the intention was to only be a 20 minute session. While this is merely an exaggeration to emphasise my point that sometimes we just fail to see when we should down tools for the day and save it for another training session.
The resilience and determination of some handlers to battle through a problem is, admittedly, quite admirable but unfortunately this can cause more creases than it irons out and could quite potentially jeopardise any future session you may wish to engage in with your dog. Having an argument over something late on into a session will cause the dog to become despondent, fearful, anxious and sometimes aggressive towards the handler.
In my early days of learning how to train dogs and build drive I was always taught to finish while the dog is still motivated. By doing this you are creating more motivation and desire to work with you and respond to you the next time you engage in a session.
While some types of pressure are acceptable to be used in the space of a training, you as a handler, must know when the necessary becomes unnecessary and whether your relationship with the dog is evolving or dissolving as a result of this.
If you feel the session is going to reach a point where it's becoming stagnant, it's advisable to dramatically reduce the level of complexity to achieve a level of success in the dog's mind and then down tools till the next time.
I, personally, always end my session with totally informal tug games and my dog winning the toy and going back in the car or in a crate, signifying the end to a positive and productive session.