XnView Software

Not sure what I am doing wrong here but most of the categories I click on come up without thumbnails, except for a few photos here and there, such as the ones in this screen shot. It seems totally random to me. Is this normal behaviour?

I answered my own question. I had moved a lot of the files so it was looking for them in a place they no longer were. Once I right clicked on my main folder and chose "show all files (recursive)" it seemed to rebuild all the categories. Thanks, me!!!