I move around images in selections in Lightroom 5 to get a specific order as I am in reportage photography. It happens - quite often - that suddenly I am not able anymore to move around images in the selection. The selections are not smart selections. Lightroom reports that the photos cannot be newly arranged (I use LR in German so I don't know what LR reports exactly in English). I can grab the image but when I want to place it somewhere else in the selection the message pops up. The only thing that helps is to restart LR. I wonder if there is another solution?
Custom sort order in LR is only supported in Library mode when a folder or collection (not smart collection) is selected. Simply create a new collection to do your custom sort from, or make sure that you have the correct folder selected.
You have to be working with a single folder.
"All photographs" and Smart Collections will work if the entire catalog references only a single folder.
Photos excluded by attribute/metadata still count for this purpose. Doing "all photographs" then using metadata filtering to restrict display to a single folder doesn't help.