Are you signed in with the same Church Account on both devices? At the top of Settings, you can see what username you're signed in as, and if there are recent sync errors those will show below your username.
Do you see your highlights and bookmarks at Study.ChurchofJesusChrist.org (after signing in)? If they're not there, it means the changes haven't synced "up" from the device where they were created. If they are there, it means the first device is syncing fine, and it's the second device that isn't syncing "down."
Samuel Bradshaw • If you desire to serve God, you are called to the work.