Network drives disappearing

I have this problem where I will log in and my network
drives will all be there due to a login script. However,
after some time, one may disappear that is not through the
login script, rather through user manager on a domain
controller. Sometimes its there, sometimes its not.
However, no matter if My Computer shows the mapping or
not, I'm still able to access it by way of a shortcut I
created on the desktop. However, I want to know why its
disappearing and how to stop it. By the way, it only
happens on laptops. I am stumped. Anybody seen this before?