Some tif files not visible in filmstrip after import ...

From raw files processed in C1 Pro 21 on Windows 10, I exported tif files (16-bit, Adobe rgb) to the same folder on my hdd as the raws.  I then found that for these tifs to appear in the C1 catalogue (and browser filmstrip), it seemed that I had to import them - which I did. 

But a couple of them out of the 90+ files remain invisible.  If I select those two files again for import, I'm told they're already in the catalogue.  And they're visible in all other imaging apps on my computer.  So why don't they appear?