Diagnosing Send Failed issues
It means that device was not able to upload data to the server, which most likely means that your server has some issue or your config is incorrect.
We've been using the same server with about 10 other devices which are working fine so I think it's a connectivity issue specific to the phone (we're using a VPN on it)
Just curious to see what the specific error might be in a log (e.g. time out, no route, etc.)
You can just open URL in mobile browser and check what the error is.
On one of my devices in particular, I keep getting "Send failed" and it's unclear why. Is there a log file that stores more details?