desktops icons disappear on several workstations in the morning but come back after a right click and refresh

We have a windows 2008r2 domain and store all the user profiles in a DFS share   \\domain\domainfiles\users\username and of course the desktops are under each username. Quite often I have users report that in the morning, most of their icons have vanished. Many just reboot to bring them back but I have found a simple desktop refresh brings them back easily. It is not a major issue but it does cause users to report to their managers that they need to "reboot each morning" or "can't clock in on time" and it gets IT some negative feedback. Does anyone have any idea what could cause this and how I could trouble shoot this? Most of our workstations are running Windows 7.