I've recently developed the same problem. I have 2 dogs - a female Lab/Shepherd that I adopted as a one-yr old and a male Pyrenees/Lab/Border Collie, that I adopted as a six week old pup. The 1-yr old was introduced into my existing flock, as my older dog had cancer. I used the older dog as a mentor, to settle her down a bit, and with just a few corrective measures to break focus when she became overly intrigued, I let her roam unsupervised with the flock. Never had a problem.
I got the pup after my old dog died, and trained him the same way. It's been 3 years and I have never had a problem. I even leave the chickens out to free range while I am away at work. I have never worried that they would be in danger from my dogs.
Then one day it happened. About 3 weeks ago, I walked to the end of the driveway to meet the school bus and when we returned, there was my 3-yr old jerk laying in the yard with a dead chicken about 5 ft away from him. WTH? I've never seen him chase them. Never seen him watch them. In fact, he's never shown any interest what-so-ever. So, I made excuses... Maybe he found it dead already. Maybe the other dog killed it (she does follow them around the yard) and framed him. Maybe it was an accident - there were no puncture wounds - he wasn't EATING it... But, after a scolding, I tied him up for a few days and worked him on a leash, 'breaking focus' again. The problem was that he wasn't the least bit interested in them and there were no corrections for me to make.
So, I figured it was a one-off thing from whatever circumstance and let him roam free, but kept an ear and an eye out for trouble and kept him inside when I wasn't home. My girls are out from sunup 'til sundown and I can't be out with them all the time, even when I am home. Today, 3 weeks later, I just happened to look out a window and spot one chicken squawking and running out of a covered work-area where the girls like to hang out and there was Pullo (dog's name). At his feet was another dead bird. This time he really got what-for. I made him sit and threw the bird at his feet. When he went to smell it, I shouted "No!" and did the Caesar neck jab. I did this repeatedly. Next I put him on his back and wrapped the bird around his neck and made him stay that way for a while. Then I tied him up again and put the bird into the burn barrel. I am so ****** and don't know what to do. The dogs are supposed to guard the chickens, and I keep 2 dogs so they can back each other up.
It's quite wild out here. If the dog is tied up, he's a sitting duck for other predators - bears, cougars, coyotes, wolves. I can't keep him that way or even penned. Summer is so short here, my girls deserve every second of freedom they can get. They're generally cooped up from November to April, and the bugs, weeds, and other yummies are just getting good now.
IF I had read this thread first, I may have tried the collar of shame, but I've already burned the carcass.
Any other ideas?