Brandy is really good about coming when called- most of the time. The problem I am having is that every once in a while she will stop and look when I call, but then either go on about what she is doing, or even run farther off. This doesn't happen on a regular basis, maybe less than once a week, but I don't want it to happen at all. I use treats and praise to reward her for coming, and she is also trained to come to a shaker can. Neither the come command nor the shaker can work when she decides to disobey. What should I do?