Are you certain that the block raising the event is being called? If the procedure raising the event is executing properly, and is not erroring, then I am at a loss as to where else to look. That is the last place that there could be a problem. Sorry I couldn't help you. If you figure it out, please let me know, as I am curious as to why it's not working.
I have created many custom events, and have gone through all the troubleshooting that has helped me in the past.
Did you try testing the subscription from the UI using the WF Admin Responsibility? How did you create the event and subscription? Did you create from the forms, or use a script/utility? If you did not create from the forms, make sure the event and subscription both have licensed_flag = 'Y'.
We had a similar problem when we were testing a few events. When the event was raised from workflow administratror the custom subscription was getting picked but when the same event was getting raised by pl/sql program it didn't get executed. After a lot of debug we found that the event key that was assumed to be a NUMBER in our sample subscription was a VARCHAR2 for few events and it was raising an exception which we didn't bother to capture in the custom subscription earlier. This was a silly mistake which took our lot of time.