If the broker's MemoryPercentUsage is >= 70% with messages from one destination queue1, then the broker may fail to dispatch messages to consumer on another destination queue2.
Messages appear to be stuck on queue2. They cannot be browsed and they cannot be consumed.
If messages from queue1 get consumed and MemoryPercentUsage goes below 70%, the messages on queue2 will eventually be dispatched to consumer.
However it could be that there is no consumer for queue1 for a longer time and then prevents other consumers on other destinations to retrieve any messages.