Missing messages across devices

I've noticed recently that partial conversations in iMessage are missing across devises; or not syncing properly. For example, today my sister sent me a message (she is an iPhone user) and I read it on my iPhone. This same message did not show up on my imac or my iPad. I sent a test message to her and it showed on all three devices. What????

I have double checked that all Send & Receive info was identical across devices, it was. I checked that each had the correct Apple ID, they did. I also signed out of all three and then signed back in. I checked this link: Link your iPhone number and Apple ID to use iMessage and FaceTime - Apple Support which was of no help. Any ideas what is going on? Are these messages just lost to the ether? Not sure why this would happen and I hate not know why 🙂 thanks.