This is going to sound silly, but the thing that's increased communication the most for us is setting a physical note out with important messages and a reminder to check the notes app on our phone. We kind of use our notes app as a message board/system group chat, and alters who don't usually communicate with each other are able to keep in touch with it. Communication, no matter what form it takes, is important! A lot of systems use a journal, a private blog, or place sticky notes. There are endless options. We've had some minor squabbles, but usually they're able to be talked out even if the headmates involved still may not be entirely fond of each other.

It sounds kinda dumb, but I had to stop placing myself above the rest of the system just because I'm the core. Communication between us is pretty poor, but we have a rule of logging at least 2 sentences in our private discord server (with PluralKit) in order to track collective thoughts, feelings, and actions. This open exchange has led to one of our alters improvement. She has gone from uncaring (and kind of a bitch) to wanting to be better

I found journaling to be extremely useful, not only does it give your alters a way to express themselves, but you can also look back and see how they are and what you've missed since you last fronted