The fact that it shows as delivered on the device is meaningless. All that means is that it made it to the Iridium satellite. Lots of moving parts to SMS delivery beyond that.
Edited to add: The fact that it shows on Garmin server indicates that the problem is downstream someplace, either on the Garmin end or (more likely) with the contract SMS gateway near the target cell phone "home" location.
There has not been a (reported) SMS outage since April 14. Open a support ticket.
