Guys, does anyone know how to fix the following issue with QuickBooks API?

My refresh token fails after a few days.

What I do now:

1. get token for production
2. save refresh token
3. before every script start, get access_token & update refresh_token with the saved token from step 2, only first time, next time I take the token from step 3

And after a few days, it doesn't work; the error is invalid_grant.