Are you sharing a report that was made by your company and not one of the built in reports. I get the drop down you're seeing when I try to share a built in report but my others show the sharing option among others that you are missing..
One other thing to try is log all the way out and back in (Ive found that someitmes if you've been logged in as someone else it doesn't always reset back to your login when you log out of their profile).