The one thing we haven’t mentioned, since you had the same problem with two different hubs, is local interference.
Zwave operates on a different frequency then, so it can run into interference from different devices.
In the US, the most common trouble sources are old baby monitors and old cordless landline phones. Also some walkie-talkies. If the frequency is in the 900 MHz band, or anywhere near that, it may cause problems.
In the last couple of years, a new source has been introduced: hobbyist drones. Seriously. Some of these do operate in the same band. We even had one community member who had a problem of this type.
Security Issue - DJI Mavic Pro Drone killed my SHM
Another possibility is architectural changes, particularly the ones that are physically close to the hub.
I once knew a family that kept their hub in the garage, bought a new motorcycle, and then ended up parking it right where it blocked signal transmission from the hub to the main house. 
In another household where they set up a new aquarium in the family room where the hub was located, and, again, it blocked signal on that side.
Usually the easiest way to start looking for problems of this type is to first make sure your hub isn’t near anything that’s an obvious potential problem and then move a new device about 2 m from the hub and see if it works there.
If the new device still still having problems, then you have to start looking for either RF interference or assume a defective hub.
I know it’s frustrating: smartthings just doesn’t really give us diagnostic tools that can help much in situations like this. 