Loosing Subnet mas

I have a Win98 system connected to a network, it is getting the IP address
via DHCP from a Win2K DNS server…I have the client setup as follows:
IP Address tab set to obtain IP address automatically
Wins Configuration tab set to use DHCP for WINS resolution
Gateway tab has correct gateway listed
DNS Configuration tab has Hostname, Correct Domain and primary and
secondary DNS servers listed in order

Our problem is that the user (who logs off nightly), will work fine for a
day or two obtaining IP address with the correct subnet mask no problem,
however other days when the user tries to log in he gets a “No network
available� message…I call up the IPCONFIG and it shows a good IP address and
the correct gateway but the subnet mask is 255.255.0.0 vice 255.255.255.192.

What could be causing the loss of the subnet mask?