Multi master

I’m attempting to operate 5 of our platforms together. Normally each platform has a single MC43FS. This project has 5 of these controllers, connected with diagnostic and master buses. If more than three controllers are powered, the headmaster fails to power its outputs. I figured that this could be due to too many application outputs. I’ve attempted to use two separate master buses but this still doesn’t seem to work. Any help would be much appreciated, this is a one off project for marketing purposes so it’s a bit out of our usual expertise.