Three things can cause this issue:
The most likely thing your missing is the action hasn't been added to your status. You need to make another process the default, deactivate the process, then look at the properties of the STATUS you want the action to work at. Click on collections, select your available action instance and then the ">" button to add it to this list. You can also add an "Action Instance" (light blue box) to your process and add more process steps after it to reach the same effect. Warning: You can't remove an action from a status and then add the Action Instance box to your process without saving it first. If you try that you will lose any changes you made since your last save.
What if you don't see the action in the list? You probably never created it. In the object designer, select Incident Management (your object) then Incident to open Incident. Then select Manage Actions and use ADD to create a action that you can use above.
What if you can see it but nobody else can? Your the admin, you can see everything (good and bad). You need to add privileges to the role or group that you want to have access. I like to add rights to roles as the users can't change those, my personal preference.
This should help you out a bit.