We only play when humans are watching.

I don't think dogs ever play unless a human is watching. Kind of like that cartoon strip where all the farm animals are busy chatting, composing symphonies and scratching complex equations into the dust with their claws. Then Farmer Joe Bob comes around the corner of the barn and they all go back to mooing and clucking and scratching for corn kernels.
I go out on the back deck, followed by the Rottweilers. I sit on a chair. They wander down the steps, putz around the yard peeing for a few minutes, then break into play. Growling, panting, chasing. Tumbling and tugging. Teeth bared, twirling, racing around the big tree. I get up to go inside; they screech to a stop and race for the door to come in with me. I've even tried leaving the door unlatched so they can't hear and crawling towards it so they can't see me.
(This no doubt looks pretty gormless.)
But they sense my withdrawal and come up to follow me in.
If I leave them outside, they scratch mournfully at the door for a while. If ignored, they lay down on the deck, put their heads on their paws in puddles of Rottweiler lips and look put out. I never see them playing unless I'm right there. I don't believe they do.

Fly update: Better. I am not allowing bones in the back yard. I have raked up almost every scrap of straw. I removed all the fly traps with their attractant. I even called the MSU extension office. They said to bring some flies in so we could determine what sort they are. I don't think I'll have time this week; they're way on the north end of town.
I'm going to call this place and ask for recommendations. I like the idea of biological control. Besides the fly-eating wasps, there's several other handy looking bugs to choose from.