Hi Guys
I know it’s old, but if someone is coming here after googling it’s very simple.
In your Auth0 application settings tab → Refresh Token Rotation, set that on, so when you do a refreshtokenasync(oldrefreshtoken), this will return a new refreshtoken in the response.
If the setting is off you have to use the original refreshtoken as the parameter in refreshtokenasync(firstrefreshtokenreturned), that was returned during the login flow.
Also, if you are not getting any refereshtoken, make sure your API as offline_access turned on.
Hope it helps someone