"share to edit" link does not appear in share menu

hi there, the option to "share to edit" for all of my prezi's was active yesterday, but is not today. even if i try to make a new prezi to share for editing, the share to edit option does not show in the sharing menu. i have the free edu account. does this occur regularly? can this be fixed? if so, how? i've tried everything.
thanks in advance