Yeah. I know it's a sport either of you play, but there is a reason that start-line stays in agility are often MUCH harder to have and keep than a stay in other situation. The reason is this - if you aren't careful anticipation of release toward highly exciting activity results in the stay degrading. Walking back to reward with food outside of trials is pretty common. Or throwing a toy BEHIND the dog, to cut the anticipation of the course being what's released to, every time.I understand this is not really an "asking for advice" situation, but I will say this with the hope that it may help you in the future..
I prefer to reinforce the stationary exercises such as sit-stay and down-stay with food rewards if possible. Delivered calmly, and passively, with minimal if any excitement. Using tug and / or playing fetch as a routine reward can have a tendency to engage a spirited dog, who in this case is already in drive, into further drive, due to anticipation and the previous reinforcement history. In other words, the dog knows full well that some form of "action" will be coming, so he is likely to be inherently twitchy and over-eager to actually *move*. Under those circumstances it's extremely difficult to succeed at staying unless the dog has had a TON of impulse control work, EXTENSIVE proofing with toys, etc.