Hi MayaStina,
I realise that this post may be somewhat outdated. However, I have chosen to post my solution this as it seems to be a common issue.
For an unknown reason, when I updated my Endnote I also lost this ordering feature.
To remedy the situation, check that your Edit & Manage Citations page is ordered by “Citations”, rather than Count or Library.
To do this, highlight a citation > Edit Citation(s) > More > Click the “Citation” ribbon.
This fixed the issue for me and I can now reorder my citations manually. I hope this works for you (and other users).
Best regards,
Dan