Does your Agility Dog Stay everywhere but on the starting line? My "over the edge Border Collie", started breaking her start line stays after about one year of trialing. I had to go back and examine the cause and what led to the deterioration of her start line stay.
As I pull pieces of the past, I realize that at first I asked my dog, Rip, to Sit-stay at the start line. But she began to lean and scoot, so I changed to a Down-stay. She did this for a while until she began to crawl and creep. Then I would walk away for a lead out, but I would continue to face her, by being face to face, that would help her to hold her position, well it did for a while. Then I started getting breakaways, she would no longer wait at the starting line, as soon as I started to step away from her side, and she would be off. That's when I filled her from trialing.
Why did her behavior get worse at the start line? Because I started to negotiate with my dog, way back when she moved from the Sit-stay, I should have made it clear that the Sit-stay was the only behavior that would bring the reward of running agility. What I mistook as, "Oh, I'm asking too much for my edgy dog to sit-stay on the line." She took as, "Mom, does not really mean what she says when we're in this fenced ring with lots of people around." So, she began to test every behavior I asked for on the line. Dogs will do what we allow.
If your dog begins to break its start-line stay, whether that is a stand, sit, or down, STOP immediately. Do not run your run. I know you've paid your entry fees, but you'll be spending a lot more time and money fixing the problem when it gets worse. For some dogs, taking them off the course is deterrent enough to get back on track. For other dogs you may have to "train" your stay in the ring, of course you also, may be called for training and excused from the ring, but it's most important that your dog know that the fun can not and will not start until the Stay stays.