If you are asking for more resources (such as cores or memory) than are currently available, your job must wait for the requested resources to become available before your job can start. Some job configurations will never run; it is important to check that the resources your job is requesting can be provided in the job queue submitted to. See the list of job queues and associated resources when choosing an appropriate queue for your job.
MSI performs scheduled maintenance on the first Wednesday of each month. You may be requesting a walltime of such a length that your job cannot finish before the next maintenance window; this will affect you if the system on which your job is queued is scheduled for maintenance, as jobs will not start if they cannot complete.
If your group's recent usage has been larger than your group's fairshare target on a particular machine, your jobs will have lower priority and may take longer to start.
Further information for a specific job can be obtained using the command:
squeue -j jobIDnumber
Here jobIDnumber should be replaced with the job ID number of interest.