Why can't I view my detailed bill online?

I'm replying to my own message with a follow-up: I called T-Mobile again, and the second representative was able to address my issue. She had me log out and log in again, and she worked magic to make me the Primary User of my account. I'm not sure how I would have done this myself, and I was thrilled that she was able to help me. She was far more capable than the first representative I spoke with. So, I guess the lesson learned is that if you aren't satisfied with how a rep is handling your question, you have the right to end the call and to speak with someone else.