Hi @12p329,
Can you recheck your request body? I just tested it, and it works every time.
Thanks,
Neil
POST https://auth.atlassian.com/oauth/token
{
"grant_type":"refresh_token",
"client_id":"<your-client-id>",
"client_secret":"<your-client-secret>",
"refresh_token":"<your-refresh-token>"
}