I have had some luck with hand editing the lensfun database. I get the exact character strings the camera/lens reports from the image information in the Lighttable view. Then I add or replace the database entry.
So, while it may look like your camera/lens is correctly shown in the Lens Correction module, if there is even an errant space or dash character from that given by the EXIF info, you will get the error message that it cannot be found.
Also, this means that the next time the database is updated, you may have to edit it, again.