Do you use the word STAY alot with your dog? Do you actually mean for your dog to stay until you say they can move when you do use it? Have you taught them what the meaning of stay is? Most people use the word stay as a sort of pause instead of the real meaning. This casual use of the word stay leaves your dog with a cloudy understanding of the term and you with a less than confident feeling about your dog doing an actual stay.
Stay means to stay in the exact spot I put you; until I say you can move. This should never be used as a casual "hold on a minute" term. Like many verbal cues we use for our dogs you must follow through and enforce a behavior so that your dog understands it to a T. Stay is a behavior that starts out very small, no distractions or great expectations. As your dog learns what stay means; you lengthen the time and then the distance between the two of you.
When I teach a stay the dog is never called out of it; the person always returns to the dog to release them. This way there is never an anticipated mixted message "did you call me?" A stay needs alot of "proofing," meaning that you add many distractions and temptations to get your dog to break a stay. That is; once your dog understands the full meaning of stay and can handle each level of distraction. There is no point in trying a new stay with your dog around high level distractions like other dogs.
If your dog does not have a solid stay then make sure the distance between the two of you enables you to return to your dog in short order. Once your dog is up and out of a stay; a game of chase is not going to help the situation. Only move on in difficulty and distance once you master each level. Choose a word that will be a release and this is the only word that gets your dog out of a stay, no matter what.
Even with my youngst dog being 7 and the girls both in the senior years I constantly proof my release word in the use with a stay. I talk to them with fluctuating tone and volume using words that sound very much like my release word. This causes them to pay close attention to me; which is what I want. That and using stay; at odd times during the day ensures a solid stay behavior when I need it.
So before you casual tell your dog to stay; consider your follow-up. Are you going to supervise, enforce and release? If not then don't utter that word, use another word that just means "hang on for a second," like "wait," and don't tarnish your very serious and very formal STAY. Believe me, you will need it someday.