Can't change assignee field in View Issue screen But can change in view screen with AllOpenIssue Filter.

Hi,
I'm using JIRA 7.0.10,
Why am I not able to change the assignee field directly from the View Issue screen of an issue, but when I can change the assignee when I open the same issue using AllOpenIssue Filter.
P.S. I am the admin, with all rights possible. Assignee field is present in Edit screen as well.