Remote desktop clients : saved password lost

Remote desktop clients : saved password lost

We use remote desktop on a regular basis ( about 40 clients ).
On the client computers we save the passwords in Remote desktop connection
( by checking the checkbox 'save credentials'). Doing this the user name
and the password gets saved in the registry.
On certain client machines, Remote desktop connection asks for the
password after a few days. In the options of RDC, the checkbox 'allow the
password to be saved' is unchecked, and we have to check it again to be
able to save the password.
We did not find what triggers the loss of the passwords nor what could be
the difference between the computers that loose the password and those
that don't.
Notes :
We use RemoteApp. We usually connect using the shortcut created by
installing the RemoteApp .msi package and not with RDC.
Most of the client computers are in remote locations and are in a
Workgroup. We are SaaS provider for an application that runs on Remote
Desktop