If I were you, I wouldn't worry too much! Many boars are very happy with your arrangement of having their own territory but shared roaming time. I assume that all your boys are still subadults in the big teenage hormones?
In this case, you may have more success with another round of trying to pair them up when they are over 15 months old and their hormones are finally settling down, especially as they haven't fallen out and had big fights. Hopefully, you can get at least one pair then. But don't try to force it; if character compatibility is not there, then you do not have a base for a stable bond. Considering how much piggies need company of their own kind (even if it is just through the bars), they can be ever so fiddly to pair up - sows aren't necessarily easier, by the way!