Sending Logs to Slack

Hey guys,
I am having trouble understanding why my instance of logstash isn't sending the files to slack. I was hoping a pair of fresh eyes may be able to find my mistakes. I am simply using an Ubuntu server and am wanting to send some of the server's own logs as a test.

     path => "/var/log/apt/history.log"

     match => {"message" =>"%{GREEDYDATA:message}"}

output {
  slack {
     url => "MY_SLACK_WEBHOOK"
     channel => "#logs-syslog"

Are new lines being appended to the end of history.log, which Logstash is tailing?

That is my understanding of how the logs are set up.

If you replace the slack output with stdout { codec => rubydebug }, you get output?