Why is it doing this?

Should it be an open circle or a solid circle?

Solid circle, and it prints solid for about 5 impressions. I’m not mashing the type or anything either.

The problem could be voids in the body of the type (due to poor casting). These voids (particularly near the face of the type) cause the face to ‘sink’ under the pressure of printing. So long as you don’t mind damaging the type, you can slice the type through the body to see if there are voids; well-cast type should be fairly (though not completely) solid.

I had the same problem recently. I was printing patterns with 36 point solid circles and half of them, in a form of about thirty, were doing this. I have a collection of a hundred or so of these, so I started replacing the bad ones with others. I could feel the weight difference in the bad ones. Some I could see the dished out surface of the type. So I put the bad ones on my saw and cut them in half. Sure enough, the bad ones were hollow inside. I was not doing deep impression just normal amount of pressure. The bad ones were poorly cast Monotype.

Thank you all, I had a sneaking suspicion that was the culprit. I’ll cut into one soon to see for sure