Thank you for reaching out to Constant Contact API Developer Support and for your patience. My team is here to assist outside software developers with questions about building into Constant Contact's API.
To the question at the end of the post, it isn't necessary to refresh the token every time it's used, only if it's expired. The two most common reasons for expired refresh token errors are:
1) When you refresh, you get both a new access token and a new refresh token. If you are able to refresh once, but subsequent refreshes fail, you may not be storing the new refresh token, and the code may be continuing to use the old one.
2) If you are starting the authentication flow over from the beginning, any previously generated tokens are immediately expired; you have to use the most recent set of tokens.
If this doesn't resolve your issue, please feel free to send us an email at webservices@constantcontact.com with your API key and the username of the account you’re connecting to so we can look into it further for you.
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