Problem importing iPhone pics to Photos app

I'm having an issue importing my iPhone pics to the Photos app on my Macbook.

I use an external drive as my Photo Library (too many pics/videos to leave on my hard drive), so I don't know if this is part of the problem.

But I notice that not all of my pictures appear as available to import to Photos. Somehow it's skipped over some of my iPhone pictures and I have no idea why. I know I could manually Airdrop the missing ones to my hard drive and then put them on my photos drive, but that's very tedious.

Why isn't Photos showing all my iPhone pics?

Thanks in advance.