You cannot manually rearrange the order of the photos in the main Grid. You can only manually rearrange the order of photos in a folder or in a collection (but not a smart collection).
If you are in a collection or folder and its not working, make sure you point the mouse at the center of the photo and begin your drag from there ... do not point the mouse at the grey rectangle outside of the photo, you can drag and drop from there.
None of those options are working for me either. This was never a problem for me in LR4. I just upgraded to CC and it seems to be an issue since then.
Ok, actually a little embarrassed to admit this, because I know better! I restarted and now it's all good. Thanks for trying to help me out!!