You´re having the same problem as I had with my three. Last Christmas, I was looking after a friend´s dog and staying at her house but spending most of the day at my house with the boys. However, I decided to house them all together for the time I was away as I thought they´d keep each other company even though two of the boys were housed separately as I had a small issue with their bond after neutering. Anyway, when I came back, I housed them separately again but thought they might be able to go out together but it wasn´t possible as they chased and nipped and sometimes fought so I had to let them out separately with my other bun who still got on with them both. I tried bonding again in the bath, supervising while out together and had periods where I went back to them being separated for weeks on end as it didn´t seem to improve or improved for a while but then they´d fight again.
A couple of months ago, I decided to proof the balcony so they had extra space to use and so they could get some fresh air and I let them all out together there. Lo and behold, they were fine together and since then, they´ve been OK. They still chase and nip but it´s more playful than anything else. The process took about 7 months of trying and then giving up and trying again but they finally got there.
So, I think you will just have to work on the two who aren´t getting on and bond from step 1 of the process again. You know your buns so you will have to supervise carefully. I started and stopped several times when I thought there was no progress but I´m glad I persevered as I knew these two love each other to bits, they´ve been together since birth. Patience is the key and let them go at their speed and not yours if you want the bond to last again.
Maybe the next time you go away, you could get someone to housesit so the buns routine and environment doesn´t alter.
I know they´ll be ok eventually again but it´s just getting there that is so difficult. Would love to see some pics of them.
