Acrobat license just deacivates from users at random

Over the last year every so often I will have a user that all of a sudden it says there license has expired.  When I login to the admin console and look at there account no license shows up, but I have a license available that was previously assigned to them.  I did not remove them and I am the only admin for my company.  Why does this happen and how do we stop it from happening?