I have had success at bonding boars, but I have also had failures.
I have had a long standing pair of boars for a couple of years now. (Toffee & Ginger)
I tried to bond another pair and whilst initially all was well, eventually they had to be separated. (Skip & Jeff)
I then tried to bond another pair and the same thing happened. (Percy & Sid)
I have over the last couple of weeks been attempting to bond Skip and Sid. These were the ones in each pair that were bullied by the others. I tried them in a run and have not looked back since. Its as though they have always been together. They have an initial run around and then settle down to a nibble of the grass. They act exactly the same as my original pair. They are now living together and so far so good. Keeping all my fingers crossed.
As for Jeff and Percy, I am keeping these as singles for the time being.
As has already been said, the personality of the piggies also has some bearing on the situation.
Good luck.