Apple Mail Smart Folders Become Unavailable

Using Apple Mail on Mac, I created some smart folders. Most of them worked perfectly well, however some of them become Unavailable - and I cannot double click on them to edit them. They are basically busted and I have to remove them and start again with a new Smart mailbox.

I think generally the criteria is to show Mail in the last few days or email date is today – criteria like that. And when I first created it, it seems to work fine but in a day or two later, it becomes unavailable and un-editable.

Any idea why some Smart Folders would work for a few days and then stop working and break? Any Way to fix these?

Any Ideas on why these become un-editable?