Does the problem affect specific parts of the network?

You might be able to identify specific parts of the network that are affected by the problem (remote queues, for example). If the link to a remote queue manager is not working, the messages cannot flow to a target queue on the target queue manager. Check that the connection between the two systems is available, and that the channel initiator and listener have been started. Use the MQSC

PING CHANNEL command to check the connection.

Check that messages are reaching the transmission queue, and check the local queue definition of the transmission queue, and any remote queues. Use the MQSC

BYTSSENT keyword of the DISPLAY CHSTATUS command to check that data is flowing along the channel. Use

DISPLAY QLOCAL (XMITQ) CURDEPTH to check whether there are messages to be sent on the transmission queue. Check for diagnostic messages at both ends of the channel informing you that messages have been sent to the dead-letter queue.

If you are using WebSphere MQ clusters, check that the clustering definitions have been set up correctly.

Have you made any network-related changes that might account for the problem?

Have you changed any WebSphere MQ definitions, or any CICS or IMS definitions? Check the triggering attributes of the transmission queue.