I think most of my friends at LCDA have seen my Boykin, Rusty, leave the ring many times. I had seen people with high drive dogs who would bark continuously until their handler would let them run the course and I just assumed that my dog would be naturally excited about agility like these dogs. For months I thought that my dog was leaving the ring because he was stubborn or he just didn't like agility. Finally I realized that I shouldn't be blaming my dog and I realized that the real problem was actually me! I wasn't giving him any reason to be excited or to enjoy agility. I would simply walk out to the field, let him off his leash, and expect him to run like a pro. At the end of the run I would tell him he was a good boy, put him back on his leash, and we were done. I wasn't rewarding him as much as I should have been, nor was I playing with him or interacting with him enough.
Once I realized that I needed to train my dog with a more fun and excited attitude, I went back to the basics. I spent a couple nights a week retraining all of the obstacles with more speed and excitement. Once I had retrained all the obstacles, I started running courses with him without correcting him or using any negative words. If he missed an obstacle, it was no big deal and we kept running. Everything we did was upbeat and happy, which allowed him to enjoy himself on the field.
I think that the best advice I can give to a handler with a disinterested dog is to keep moving! You don't want to get too far ahead of your dog or you won't be able to handle him as well, but you can't just stop or your dog will stop too. The number one reason why my dog has left the ring is because I stopped suddenly to wait for him to finish an obstacle, or I'd make him repeat an obstacle he missed. Since most dogs who are not high drive (at least in agility) run slower than their handlers, it can be tough to keep moving without leaving your dog behind.
If you run faster than your dog, then you can take smaller strides instead of full-on running. This way you won't get too far ahead of your dog, but he will still see you moving. While your dog is in a tunnel or the chute, pause for a second and then start running again once you see him getting to the end of the tunnel. Every dog runs differently so you have to figure out little tricks to keep yourself moving without getting too far ahead of your dog.
However, what you do before the run is just as important as what you do during the run. If your dog would rather smell the grass than play with you, then it is best to get him out just a couple of minutes before his run. Give him just enough time to go to the bathroom and stretch his legs. Most dogs are very excited to see and play with their owner when they first get out of their crate. The idea is to get him to the start line with the same excitement he had when you first opened the crate door. As you are walking to the gate, keep your energy up. Talk to your dog excitedly and walk quickly to keep him happy and interested in you. If your dog senses that you are excited, then he is going to share the same excitement. I like to do a couple of quick tricks with my dog before we enter the ring, like hand touches, spin, and speak. If your dog likes to play tug with his leash, this is another great way to keep your dog interested in you before a run.
I am happy to say that the dog I run today is a much happier and excited dog than the one I ran 5 months ago. He still occasionally leaves the ring or stops to smell the ground, but he is nothing near as bad as he used to be. The most important thing to me is that he now runs with me because he thinks it is fun and knows that he is going to get lots of praise and not because I chased him down 3 or 4 times and carried him back into the ring.