The squeue -u <username> command will show you all of your the jobs that have been submitted in the queue. The last column will tell you the reason the job is not running. Or, once the job is running it lists the node(s) the job is running on. The most common reasons include

1. (resources) - the necessary combination of nodes/cpus for your job are not available.
2. (priority) - there are other users in the queue with a higher priority and will therefore be scheduled first.
3. (dependency) - The job will not start until the dependency specified by you is satisfied.
4. (QOSsomething) - This tells you which limit the job is exceeding in the particular QOS.