John here are some thoughts and suggestions that come to mind. I tried these with success. I'll be frank, i've stolen these from great trainers.
1. Always remember that even if the problem is not being exhibited in training, that doesn't mean you can't work on it. Increase your standard on line manner issues. For example, if he's totally focused and perfectly still on the mat, is he really focusing on your leg or totally focused on the field? You may need to simplify the marks and insist that he check in with your leg between marks in training. Begin to teach him that sitting there is not enough, he needs to be constantly aware of you're presence and leg position.
2. Put the dog in the field while you run all the other dogs. I often run the dog with line manner problems early in rotation then take him out to a position in the field where he will be tempted by a mark and make him lay down and watch all the other dogs run it. Make him own laying down. a collar works best for this. Demonstrated in the Farmer Problems video
3. Teach the dog to heel backwards. I believe there are two reasons for doing this.
A) the dog learns that the only proper position is next to your leg, and their will be a correction for not sticking there. It also teaches something that dogs do seem to have issues with, backing up. A lot of dogs don't know how to backup so you'll see them swing finish to get back in position. That really messes up the marks.
B) Teaching a dog to heel backwards gives you personal yard time to establish a tighter team bond. You're providing additional time where you are the leader, your challenging him with new skills and you will find good results from this regardless. Also find and design little yard control games wiht him once he's heeling backwards. If he doesn't two side heel, teach him this. Keep challenging him mentally and keep him relying on you for direction.
4. Review your whole running procedure. In training we tend to not focus so much on control from the truck to the line. Control starts at the truck, develop good habits and a routine of control right from the truck. See the Rorem handling video
5. Teach the dog to lay down in the holding blind. Establishes control
6. Use a choke chain in training and use it to correct for heeling issues. This gives you a tool at the FT while moving from holding blinds to remind the dog who is in charge. Just the sound of the chain sliding should remind the dog.
7. In the trial watch the test, at times there will be an advantage in which way to come out of the holding blind. That advantage goes away if the dog is out of control, so therefore watch the dog. If he's already showing signs of independence and wants to come out of the blind on a certain side, switch it up, make him come to you and go out the other way. Establish the control.
8. Use a mat at the line in training. Make him adhere to the mat, control and correct for any mat infractions
9. In training when he is waiting in the blind step back 5-10 feet and make him come to you. Do not get in the habit of just stepping out of the blind. Use this as a control tool in the test.
10. Increase the difficulty of the factors influencing the line manners in training and decrease the difficulty of the setups. Use this to work on new control items like laying down the holding blinds, heeling backwards etc.
Hope this gives you a few ideas....
/Paul