Some of this seems obvious, but it is a good checklist.
1. Que Manager has not been started 2. Listener not started 3. Initiation queue not started or created or attached(usually optional depending on set up) 4. Listening to the wrong Port or IP 5. Firewall stopping traffic to Port or IP 6. Queue Manager not created on destination 7. Not authorized to Queue manager and/or channel and/or queue 8. Trial MQ copy expired. 9. Wrong Queue manager name 10. Wrong channel name or password or queue or queue type