My Eraser Stopped Erasing Everything Despite 100% Opacity, How Can I Fix This?

I'm using a background eraser and eraser to refine the mask, and the settings seemed to change suddenly, but I can't figure out how or where. Granted, this is also happening with the regular brush as well, when I try to restore an area that was over-erased. Instead of the perfect alternating squares there are circles within the areas of the brush. Please help, I'm a very light user and need this for a project. I have included a screenshot showing the difference.