Moving Pictures within Photos Library?

A photo will bounce back, if you try to add it to an album, where the photo has already been added to.

And you cannot drag photos to smart albums or some of the albums that Photos is creating automatically for us.

If you have been trying to rearrange the sequence of photos in an album and to drag them to a new place in the album, try to drag them in several, smaller steps. Photos does no longer scroll the album, while we are dragging, to allow us to drag over larger distances.

And you cannot move photos between albums, only add photos to additional albums. But when you add photos or videos to an album, the photo will not be removed from the album where you have selected it and dragged it from.

And you can't change the order of photos in Library View, or in any of the views at the top of the sidebar. If you want a "custom order" of your pictures, put the ones you want into an Album. You can select pictures you want and then choose "File>New Album With Selection." You'll get a new album that you can name as you please. Then you can sort them or drag them around.

As léonie said, the window does not scroll automatically-- on my Mac Air I drag a picture a bit, then keep my thumb down while I scroll with two fingers. It's like the game "Twister" but with fingers.

This screenshot shows the sort options available in the various views of the library:

Where are you trying to move or copy it from and to? Not all combinations are valid.