This is my second PCB ever, so I'm still constantly learning things. Here's something I didn't realize until the boards showed up and didn't have continuity -- I had multiple, unconnected ground planes because my board was designed to be barely wider than the ItsyBitsy. I now realize that all I had to do was count the red copper fill connector thingies that I've numbered below.
If the board were 1/8" or so wider, this wouldn't have been a problem at all. So, now I have a wire on the back connecting the top and bottom ground planes. There are actually three ground planes total, with the third being underneath the IB. I don't need that one, so it's not getting a wire.