Render PNGs are mostly black

I am rendering a short (30 second) animation and I let it run for a little while to make sure everything was doing okay before I set it to run for hours, and when I came back and checked the renders, most (26/30) of the PNGs were completely black. I don’t know what is causing this, please help.

Here are my render settings:

It works fine when I render any frame as a still image, but this happens when I render as an animation. Any help would be appreciated. Thanks!

I’m not sure: i might have heard something about the min samples better not be 0 ???