Most plugged zwave or Zigbee devices are repeaters, but it is up to the manufacturer to decide for sure. That said, I’ve never heard of a Z wave device which wasn’t a repeater except for some security items like Sirens where they don’t want the device tied up repeating a message for some lightbulbs somewhere.
With Zigbee, there are some mains power devices which are not repeaters, such as the Sengled lightbulbs.
No, they don’t Queue them in that sense except for Messages intended for locks which have a different subprotocol called “beaming.“
For other devices, the hub will make three attempts to send a message through, one right after the other if it doesn’t get an acknowledgment, but that’s it and typically happens in just a few seconds. But after that it gives up.
It’s hard to say exactly what’s going on because the smartthings platform can definitely be flaky sometimes, but my first guess would be range. It’s often challenging to get signals out of basements and garages because of all the cement and water pipes and metal that can be present. so as a long range plan, speaking as a former field tech, my first recommendation would be to move the hub out of the basement or you may be facing this over time with many different devices. . I’m not saying that will definitely fix this problem, I’m just saying that it’s a good idea in any case.
(And then the fact that you’re trying to get out to a greenhouse means you’re going through at least one exterior wall which also blocks significant signal.)
See the following. Start with post 11 in that FAQ (which this link will take you to), read that, then go up to the top and read the whole thread. (The topic title is a clickable link)