Inconsistent margins

Hi, I’ve been using this theme for about a week now, and really like it. But I’m run into a small snag. Some of my posts look great, while others are set way off to the left and get cut off.
Everything was fine until my post of April 6. It posted fine, but then the posts before it (April 5, etc) were all shifted over to the left. I made another post today, and both it and the April 6 post still look good, but the same issue is going on for April 5th and all of the earlier posts.
If I open any one of those earlier posts individually, they look fine, but when I look at the blog as a whole, they’re off.

https://www.jellyfishfloat.com

I haven’t edited any of the original files for the theme, just adjusted the colors, etc. through the “Appearance” tab on the Dashboard.