Fleetsmith will only escrow FileVault 2 Recovery Keys for devices Fleetsmith encrypts.
Devices encrypted prior to Fleetsmith by another source—or devices with FileVault 2 manually turned on via "System Preferences"—will have a "Managed" Encryption status on a device's page and no Recovery Key will be available in the Fleetsmith web app.
When the "Show Key" option is available, that means you can access the escrowed Recovery Key via the Fleetsmith web app. (This can be really helpful if a user is ever locked out.)
You can re-encrypt your device through Fleetsmith by following the steps here: https://support.fleetsmith.com/hc/en-us/articles/360024569274.