The following tips and commands (while not an exhaustive list) can be useful when diagnosing problems in NATS JetStream clusters:
Troubleshooting tips
Look at nats-server logs. By default, only warning and error logs are produced, but debug and trace logs can be turned on from the command line using -D and -DV, respectively. Alternatively, enabling debug or trace in the server config.
Make sure that in the NATS JetStream configuration, at least one system user is configured in this section: { $SYS { users } }.