Surely, we know that our "auto-follow-back bot" hasn't follow-backed all of them. But our "auto-follow-back bot" has follow-backed all of the not-yet-follow-backed accounts.
So, we think that twitter doesn't accept any follow-back-requests that our "auto-follow-back bot" does. We infer from some cases that twitter doesn't accept the request for "suspend accounts", "accounts which similar to account that you blocked".
Aug 16, 2019: in Aug, 2019, we did a little experiment. Then, we found that Twitter didn't respond to our 4bot's follow-back requests for accounts that Twitter considered "purchased followers."