If you're talking about places on earth it would be for example you're in an air plane or maybe you're on a mountain. Also if you have a lot of pressure on you that would be an example too. If you're talking about in your body then you would have the most difficulty in your chest or in your nose. If something heavy is on your chest it would be hard to breath. Or if you have a clogged nose. I hoped this helped you :)
~ Ria