Hmm birds, rabbit and all manner of exciting things could be in that grass, in any case exploring that grass is WAY more exciting that running to a stupid blind pile. Either that or the dog has reached a wall in regard to distance, and he needs to be stretched out. I'd probably start running some lining drills, (W pattern, 5 Stake) with stakes, thru cover, so he KNOWS where he needs to run to, regardless of distance and then begin correcting him for choosing to screw around when he's supposed to be on a back. I'd handle this the same as popping, (no whistle) direct pressure BACK NICK BACK, lets do it again, he thinks about investigating BACK NICK BACK; again a little hesitation BACK. Let's see how long he's thinks investigation of grass is preferable to continuing to run back. Once you setup this correction in a pattern field, you can transition it to a straight back when he gets ideas of stopping on cold blinds. The thing with correcting hesitation-popping like this is you have to always be ready for it, if you miss the timing dog doesn't get the correction when he needs it and he doesn't learn to keep going. The time to correct is when he's considering stopping, not after he's already stopped. If you wait until he's stopped blow a whistle then correct; the dog will not understand why he's being corrected, or think he's being corrected for not sitting on a whistle. You'll either get a faster sit, or a confused-buggy dog .