Coming up the middle

[ragtimedog]

I read where this topic was touched on some time past, but it wasn't exactly my situation. I have a 5-yr-old dog who does a pretty good job around the farm. But when I take him out with the whole flock (7 ewes and wethers), he will focus on any that might be running and ignore the rest. I had his uncle who used to do this, but I would lie him down and make him look back. He eventually got the idea, but this guy doesn't seem to get it. He looks great if we're working 3 Katahins who have velcro for hair, but my flock is pretty dog worked. Not that any are super heavy, but they may split or wander off and he'll come right up the middle (after a beautiful wide outrun) I can make the Ahhh noise and he knows exactly why and goes back to pick them up, or I can make him look back, but he seems to focus on the ones that move the fastest (that time around.) Is there anything I should be doing to get the idea across?

Thanks,

LP