ntfy doesn't ping on boot

I'm running ntfy to tell me when a raspberry pi has rebooted successfully. I have it carrying out some other tasks (pinging network resources to make sure they are also up and running, since they will take longer).

This same script runs ntfy "twice": first, immediately upon running the script.
Second, once the script successfully pings another network resource.

The first ntfy message never sends, but the second one does.
Is this a boot priority issue? What would cause ntfy to fail the first message, but not the following one, about a minute later?