If you're doing your stretching four times per day and applying a good firm tug, you will get results unless there is a yeast infection. That is usually accompanied by cracking of the skin, so it's probably not a problem in your case. I suspect you just aren't putting in sufficient time. You may also get better results by pulling the opening apart with your fingers. The How to Stretch page has illustrations of what to do.
At this stage, it's too early to detect a frenulum problem.