HI Mark,
Everytime I select my photos, I click on the last photo then press and hold the shift button and drag them to the time line. It always works for me. If that doesn’t work try naming your photos with a letter and numbers example: A001, A002,and so on. I work with thousands of photos and I just drag them as stated to the timeline. I always keep the original embedded file number, when the photo was taken, what aperture, shutter speed and what camera you used for reference.
Good Luck and Happy Shooting
Ralph Hajik