A Session May Not Be Unlocked When...
In some cases a session may not be unlocked using the Unlock task. Occasionally, you cannot open a locked session as View Only (see section below).
A session is locked if another user is currently editing it in ArcMap. The session cannot be unlocked until the user closes it.
A session is locked when another user owns the session. The session may be unlocked by using the Change Owner tool. A user must be the current owner or an administrator to use this tool.