Arghya's suggestion is correct but there are more options that can help you.

1. You can set flush_mode to immediate in order to force flush or set or set additional flush parameters in order to adjust it to your needs. You can read more about it here: Control Flushing.

2. You can also consider using SIGUSR1 Signal:

Forces the buffered messages to be flushed and reopens Fluentd's log. Fluentd will try to flush the current buffer (both memory and file) immediately, and keep flushing at flush_interval.

Please let me know if that helped.

Increase flush_thread_count parameter

<match test>
  @type output_plugin
  <buffer ...>
    flush_thread_count 8
    ...
  </buffer>
  ...
</match>