Using Filebeat v1.2.3:
In case of error (no correct ACK received), filebeat retries to send the frame but before, the process is stopped during a computed duration: backoff (that is exponential until 1min) . But after an ACK is sent (no more problem) the value is not reset. So in case of new error, backoff will be probably 1min (until a filebeat reboot).
Maybe we can reset backoffCount that is used to compute backoff? check: https://github.com/adioss/beats/commit/793065013faedfaff030641988657b33c4076df4