I have been working with the gallery a bit and have had this occur on multiple occasions. I will write a new blog, give it a name, and save it. I go to upload multiple images and they appear in the gallery. For example, gallery will say "Gallery (25)." I click "save all images" at the bottom of the gallery page and decide I need to work on something else. So I "X" out of the images screen.
I later return to the SAME post and now my gallery is (0). However, my images are in my media library but I can't get them to appear under the Gallery tab so I can sort them and insert them into my post as a gallery vs. individual photos.
Can you upload photos in the same post then go back later and work on this without having to use my work around below?
The only way I've found to fix this is to go back and re upload the same photos so they will appear back into the gallery. Then this time, I do everything I need to do in one sitting and do not close the window until I'm done sorting the photos in the gallery and hit insert gallery. It works now.
Even if I delete all of the previously uploaded photos, WordPress will still rename my new uploaded photos with the appendages -1. If I don't go back and delete the other photos, then I will have duplicates of large batches of photos which is just taking up valuable space.