Getting error on refresh token

Hello, I have been using the Airtable API and having some problems in that.
First of all, OAuth access token is getting expired before 60 minutes. When I try to refresh the token using refresh_token it stays this:

{
"error": "invalid_grant",
"error_description": "Invalid token."
}

If I refresh the token frequently, it works fine. But it is getting expired before 60 minutes (as written in docs) and after it expires, I am unable to generate token again. PLease Help! Thanks