I wrote this in another message:
I physically disabled one of the devices in my dashboard list from sending and noticed that the next item in the list no longer has its messages reach the endpoint even though they are marked as received at hologram. All of the messages from the single device fail to reach the endpoint. I am thinking there is some sort of bug in your infrastructure. The device for which messages are not being delivered is 166576. They were being received previously.
I am opening this duplicate specifically because I have had this issue before with this forum where the first message is read but follow up messages are not read. This was raised before in: