Hi and welcome.
It's not necessarily something you are doing wrong.
There are so many factors involved in taking stage-lit pictures within a dark surround.
Do you use flash? Probably a waste of time trying.
You can do two important things: first you can force the iPhone's camera to focus on the stage, and when so done, you can increase or diminish the exposure before you take the photo.
To do this, set up your shot, hold the phone steady and press firmly on the subject. That selects the focus. Now you will see to the right of the box, a semicircle in yellow. This is the exposure setting - up or down - till you see an image which captures what you're after. Then touch the exposure button.
Of course, you can also post-process the images; either on your iPhone or on your computer in, say, the Photos app.
If I can be more specific, please ask.
Ian