Rabbitmq input plugin does not acknowledge after idle time

I am running into an issue with logstash not reading from a queue after an idle time. If I load the que up with messages constantly, everything works fine. If I idle the queue for more than 10 minutes or so (varies) then the messages go into an unacked state from ready. Looks like Logstash has read the message but for some reason is not acknowledging receipt. Any ideas? Is this something to do with RabbitMQ or logstash?

My configuration:

rabbitmq {
    type => "rabbitmq"
    codec => "json"
    durable => true
    ack => true
    queue => "<queue name>"
    threads => 3
    user => "<user>"
    password => "<password>"
    host => "<host>"
	port => "<port>"