I had the same problem due to my system didnt have much in the way of co consciousness. when ever I dissociated into an alter I became that alter's sense of agency... but we discovered something interesting...
sometimes even with out co consciousness things can "filter" through. kind of like how when I dissociate the act of my dissociating filters through to the alters taking control.

my treatment provider explained it this way....how do the alters know when they have to take control... by dissociation. somehow they know when each other or the body born can no longer be in control... even with out co consciousness .....something... is filtering through in order for them to take control or as some people call it switch....it may be a feeling or a need (some sort of trigger causes a person to dissociate, this in turn filters through to the alters to let them know when to take control)

Rather than trying to teach an alter everything is ok, use my systems filter system.. i do everything to mother\self nurture while aware and that will filter through to the alters.

example when storms were happening I would switch into rainy. how did rainy know she had to take control? by my dissociation during storms. so something was filtering through to her ...my feelings, my emotions, my triggers, what was happening in my life like rain storms... even without my and her having co consciousness abilities with each other things were filtering through...

I began setting things in action by talking to her knowing that what I was saying may not reach her. I began curling up in the same spot in the house with a blanket and thinking and saying how comforting this feels. I held toys and talked \ thinking about how comforting this was. I talked \thought about how doing these tings could help during stormy nights...I did all this self nurturing\mothering myself when I was not triggered, not dissociated in hopes that some how some day if would filter through just like what ever it was that caused me to switch into rainy did..

one day after a rain storm I noticed the stuffed lion in the garbage and the stuffed bear in my bed after a rain storm. ok it filtered through that there are now stuffed animals in the house and rainy doesnt like lions, no more lions.

ever time it filtered through to rainy how to self nurture by my leading by example rather than trying to get through to her\ teach her what wanst a part of her sense of agency.

my point...maybe you can try this filtering where while everything is ok you do what you would like this alter to do. and over time it may filter through to them.