 they have expired.

 <p>
 A durable subscription will continue to accumulate messages until it is deleted using the <code>unsubscribe</code> method.

 <p>
 This method may only be used with non-shared durable subscriptions. Any durable subscription created using this
 method will be non-shared. This means that only one active (i.e. not closed) consumer on the subscription may exist
 at a time. The term "consumer" here means a <code>TopicSubscriber</code>, <code>MessageConsumer</code> or <code>JMSConsumer</