This is very poor design. Aside from being unintuitive, it's not updated frequently. For example, if you free a license and return to an unlicensed document, the tab does not automatically appear. Refreshing the page does not help, nor even does opening a new session (e.g., in a new browser). When will I be able to add licenses to this document? An hour? A day?
A license may go into quarantine for up to 24 hours when you remove the user. Length of quarantine depends upon the how long it has been since that particular user last had a session. If it has been less than 24 hours, it will go into quarantine for however long it takes for it to have not been used for 24 hours.
Also, I get myself every time on the following: After removing the assigned user and clicking on apply, I forget to reduce the number of CALs allocated to the document. The license is not free for other documents until I remove it from the original document.
Hope this helps. I'm unable to attach a screen shot of the second thing.
Assuming that the licenses isn't quarantined; i've noticed that using the Update License from Server button (System -> Licenses -> QlikviewServer -> Qlikview Server License tab) refreshes itself and makes licenses available again to assign.
I know - it doesn't really make sense that they aren't freed up on their own, but that should help you.