Checkboxes - NOT tasks.
But I've figured it out — although it's weird. I was using Shift+Ctrl+C to create a checkbox. And it does. But it's not green. So apparently Evernote doesn't recognize it as a checkbox. I'm now using the Blue + (insert) icon and selecting Checkbox instead and it's working. I tested using the insert checkbox before creating this thread. But I was inserting it directly above the other non-active checkbox, and as soon as I pressed Enter, the active checkbox was converted into a non-active checkbox.
Anyway...I now know to always use the active checkbox if I want to be able to use the checkbox filter.
Thanks