Mapped drive not persistent

My client is running a workgroup and information is shared through a peer-to-peer sharing solution, i.e. no central file server. All the PC are loaded with XP Pro SP 2. 3 PC's (2 x desktop and 1 x laptop) maps to a 4th PC for a certain application. The desktops retain the mapping but the laptop keep on loosing the mapping, i.e. the user must remap the drive every morning. The share is created via Explorer/map drive with the Reconnect at logon ticked.
Any ideas what may be causing this and how to fix it?