As a person who has spend thousands of hours troubleshooting stuff I would say that the first thing you should do is try the most simple solution first. The most simple here, just make sure the lens is clean. My wife gets crap all over hers all the time and is constantly sending really crappy pictures. I tell her to wipe her lens and things change usually. It totally may not be a problem at all, but give it a shot.
Also, if you tap your screen it should come up with the green box around what you've focused. If you see red, it's bad. Again, simplicity is typically where people forget to go. Solving a math problem for 3 hours and you're pulling your hair out you just need to look back and realize you copied the thing wrong from the book... GOOD LUCK