Certain photos not importing

I recently migrated from Aperture to Lightroom on my Mac. For the most part, the process has gona smoothly, but I'm discovering that certain photos aren't importing correctly. I've solved one proboem, but not the other:

1) In some folders, by sheer coincidence, I have pictures taken with two different cameras with the same title (i.e. IMG_1111), which is possible because they have different filetypes (JPEG and RAW).

- SOLUTION: These photos became findable and importable when I modified the titles of all the JPEG images.

2) I found another folder without the problem above in which there were 2 images that just didn't present as importable. They were still visible in the import window, with a light grey border that distinguished them from the already imported images, but they were impossible to import. I was able to fix the problem with the same solution above, but this time I don't understand WHY the problem occurred in the first place.

- UPDATE: I had "Do not import suspected duplicates" selected. Problem solved.