In this case, there are several things that you need to check in there.
1. Please make sure if you have been registered as user, you can check it out on OneSignal All Users Dashboard. Also, you can check the category that you already subscribe on Tags column.
2. Then, when you updating the post, make sure if you have been choosing a post that has the same category as you post category that you subscribe. Also, don’t forget to check Send notification on post update option.
If you still getting a problem in there, please try to make testing sending on . You can find this option on Dashboard > Message > New Message.