I worked with pigs for 8 years and several of the suggestions do indeed work. Sometimes a boar and gilts that have been together for a while just don't bother any more so separation and reintroduction can be a good idea. If it's the gilts that are not cycling, some kind of stress can get them going, which is where taking them for a ride comes in. One of the places I worked was a breeding centre. If we had a gilt that wasn't coming on heat we'd
sell her, with a guarantee to the new owner that she would be replaced if she didn't cycle. We never had to replace one for that reason. The truck ride worked every time!