"No photos to import" when using Lightning to USB Camera Adapter with DSLR

I have the Lightning to USB Camera Adapter for those times when I need to send a photo from a location but can't carry a full laptop. The Apple software doesn't make it easy to choose from the many images on my camera's card but it does work. At least until yesterday, when I needed to depend on it. All the iPhone would report, after connecting, was "No photos to import". I tried the same card in a different camera and restarted both cameras and iPhone with no success.

I had been shooting a huge amount of photos on that card and wondered if that could be the issue. I later found that if I reformatted the card and took a couple more photos, all worked fine again.

Without further testing I have to assume that there is a maximum number of files the connector/iPhone will handle or there was an issue with the format of the card. (which worked flawlessly in the cameras and in a card reader on my computer)

Anyone else see this issue?