Samples work like a champ, my own terrain brush doesn't work

I’ve scoured every place I could, and I must be doing something wrong, but I don’t know what. I’ve followed what instructions I could find - compared the xml files of the sample to what I’ve done, checked screenshots of what I could find on the internet, but it’s just not working for me. Does anyone have any idea? This is the simplest example I can come up with that shows the problem. In the tiled window, I’m just drawing a circle. I’m sure I’m doing something wrong, but I don’t know what it is.

Judging by your second screenshot, your main problem is that your tileset is missing the inner corners for the grass. This means it won’t be able to deal with most scenarios it runs into as you try to paint it.

That did it. Thanks a lot! Now I understand how Tiled is intended to work.