If you need to select a connection that is not displaying there are two things you can do to workaround the issue.
Option 1: Edit that connection and change the name so the connection will appear higher in the list.
Since the drop-down displays in alphabetical order, placing a 1 in the front of the connection name will move the connection above connections that start with the letter A.
Option 2: Delete any unneeded connections that are no longer used to shorten the list of connections.
If you run into any issues or have additional questions, then please open a case with TIBCO Support.