What is the reason that requests which were previously working might now get stuck in a "submitted" status?

CA Service Catalog 12.9 and higher

One of the reasons for this is the jms corruption.

To Rectify this do the following :

1. Stop the service catalog service 

2. Please move the content of jms-data (KahaDB files) folder to a different folder outside of service catalog folder 

You may find the jms-data folder under …\CA\Service Catalog\logs\ 

3. Make sure to have enough space on the Drive where Service Catalog is installed 

4. Start the Service Catalog Service 

The entries in the view.log is as follows :

2017/03/03 03.48.44.099 WARN  [ActiveMQ Broker[brkr-563A66F193204BF4AFC3ECC561ADA7D3] Scheduler] [Topic] Failed to browse Topic: usmGlobalTopic

java.lang.ClassCastException: java.lang.Long cannot be cast to org.apache.activemq.store.kahadb.disk.journal.Location

at org.apache.activemq.store.kahadb.disk.journal.Location.compareTo(Location.java:29)