I noticed that if you do not have thumbnail, medium and large sizes checked when rebuilding the thumbnails you will be unable to insert those sizes when editing posts/pages.
I chose to just rebuild the size I needed since the other images already existed. When I went to edit an existing post, clicked on 'upload/insert' photo, went to gallery and clicked 'show', the thumbnail, medium and large radio buttons were not selectable as if those images no longer existed though they are still in my uploads folder. I tested by selecting thumbnail next time I rebuilt the thumbnails and when returning to the gallery I was able to select thumbnail.
Is there a way to fix this? I like the idea of selecting which sizes to rebuild, but not worth it if it's going to affect my ability to insert photos in posts/pages.