[Plugin: Co-Authors Plus] Certain authors consistently disappear upon save draft or update post

Hi,
thanks for the plug!

Problem:
Certain authors I add to a post ‘just won’t stick’!
I add them, save the post, and they’re gone again. The odd thing is, that its the same usernames across the whole site. I have tried deleting and re-importing the users (and everything else). I cannot recreate the problem on my dev server though.
I’m running a plugin that allows the creation of additional user ‘roles’ as well as capabilities magaement. Might be something to do with this.
the major plugins I’m using that may be at fault: Buddypress, Capabilities manager.
Thanks

http://wordpress.org/extend/plugins/co-authors-plus/