First thing to do is find someone with an android (you must know someone) and use the android DT app to protect your xNT before you go writing to it.
Second, it’s probably not working because either you have swelling (increases distance between tag and reader) or most likely just aren’t placing the phone in the proper position and orientation in relation to the tag for it to get good coupling. To get a read with the xNT on almost any reader, it is critical that you understand exactly where the antenna loop is on the reader and orient the tag perpendicular to it.
I don’t have an iPhone so I’m limited on what I can tell you. On Android, Taginfo will recognize a tag whether its got data on it or not. I would imagine the iPhone is the same.