Okay, I'm not as smart as these guys so maybe I can help.
1. Did you remove the sticker from the camera?
2. When you are in "manual focus mode" you do all of the focusing using a slider on your phone's screen. Have you tried that?
3. When you are in "auto focus mode", you have to touch the screen or hit the "center focus" button. I have mine set to C1. Have you tried that?
4. You do remove the plastic dome from the Mavic before you fly, right?
5. You do remove the gimbal lock before you fly, right?
6. You have made sure that there isn't a finger print or smudge on the camera lens, right?
7. Make sure that you close all other apps on your phone when you are flying. Especially since it's an older model.
8. Make sure that you have a good connection and the micro USB port on your phone isn't damaged or dirty.