There is a timeline of 8 hours from the time you posted a particular post. After that you can't delete or edit it.
I had the same issue. I did a post and I didn't see it so ended up posting the same comment 3 times. I went back to delete it just moments afterward and guess what. It would not allow me to delete. My big question is why does the delete not work properly?
You can't delete a post if it's the first post in a thread because it would end up deleting everyone else's post within that thread as well. But any post after the first should be able to be deleted just fine.