@Silverbirchleaf I had that exact problem! Twatdog has always been a bit odd about the boot (I think it's the automatic tailgate) but he has always gotten in it. A few weeks ago I got him into the boot easily enough and then he got spooked by something and he launched back out (and the rest of my gaggle followed). I managed to drag him by the color and shove him into the back seat to get him home, but it was a nightmare.
I tried for a day or so to even get him to go near the boot, but he was having none of it - even food couldn't tempt him which given he's a Goldie is quite something! In the end, I managed to get him into DMums car, but she doesn't have an automatic tailgate (you have to actually close the boot rather than press a button).
I finally got him into the car by just repeatedly shoving him into the backseat (I could only do it by going behind him and walking him on his hind legs, with me holding him up from behind with my arms wrapped around his chest, 5-10 meters toward the car). I actually got the idea watching someone wrangle their dog into the car when it refused. After a few days of doing that, he clearly decided the indignity of that was unacceptable and started jumping in happily and sitting down.
I haven't tried in my boot, and I won't. Whatever spooked him, it happened when the boot was closing so I think he associates the 'spook' with the noise of the boot closing. So he goes on the back seat, with a hammock and I clip him in. I think a lot of people probably wouldn't have been quite so 'forceful' in making him man up as I was - I'm certain a good behaviourist would have a fit tbh - but ultimately, he has to get in the car whenever I go into the office, so not going in the car is never going to be possible 🙁So I had to be a bit more forceful than @CoubousAndTourmaIet or @SpanielsGalore have had to be with their dogs. It's done him no harm, and he's fine now, but probably not an approach that would work with most dogs!
@CoubousAndTourmaIet people who walk their dogs by roads off the leads are insane IMO. Just because a dog does walk to heel perfectly, doesn't mean they won't ever get spooked! I'm glad Brie enjoyed her walk ❤